The Mass or Liturgy is the central act of divine worship in the Catholic Church.  The term "Mass" is derived from the Late Latin word missa (dismissal), a word used in the concluding formula of Mass in Latin: "Ite, missa est" ("Go; it is the dismissal").  "In antiquity, missa simply meant 'dismissal'. In Christian usage, however, it gradually took on a deeper meaning. The word 'dismissal' has come to imply a 'mission'. These few words succinctly express the missionary nature of the Church" 
The Catholic Church sees the Mass as the most perfect way it has to offer latria (adoration) to God. The Church believes that "The other sacraments, and indeed all ecclesiastical ministries and works of the apostolate, are bound up with the Eucharist and are oriented toward it."  It is also Catholic belief that in objective reality, not merely symbolically, the wheaten bread and grape wine are converted into Christ's body and blood, a conversion referred to as transubstantiation, so that the whole Christ, body and blood, soul and divinity, is truly, really, and substantially contained in the sacrament of the Eucharist, though the empirical appearances of the bread and wine remain the same.  
The Roman Missal contains the prayers, antiphons and rubrics of the Mass. Earlier editions also contained the Scripture readings, which were then fewer in number.
The Lectionary presents passages from the Bible arranged in the order for reading at each day's Mass. Compared with the scripture readings in the pre-1970 Missal, the modern Lectionary contains a much wider variety of passages, too many to include in the Missal.
A Book of the Gospels is recommended for the reading from the Gospels, but, where this book is not available, the Lectionary is used in its place.
The Eucharistic celebration is "one single act of worship", but consists of different elements, which always include "the proclamation of the Word of God; thanksgiving to God the Father for all his benefits, above all the gift of his Son; the consecration of bread and wine; and participation in the liturgical banquet by receiving the Lord's body and blood". 
Within the fixed structure of the Roman-Rite Mass outlined below, the Scripture readings, the antiphons sung or recited during the entrance procession or communion, and the texts of the three prayers known as the collect, the prayer over the gifts, and the postcommunion prayer vary each day according to the liturgical season, the feast days of titles or events in the life of Christ, the feast days and commemorations of the saints, or for Masses for particular circumstances (e.g., funeral Masses, Masses for the celebration of Confirmation, Masses for peace, to begin the academic year, etc.).
The Start of Liturgy
The priest enters, with a deacon, if there is one, and altar servers. The deacon may carry the Book of the Gospels, which he will place on the altar, and the servers may carry a processional cross and candles and incense. During this procession, ordinarily, the entrance chant is sung. If there is no singing at the entrance, the entrance antiphon is recited either by some or all of the people or by a lector; otherwise it is said by the priest himself. When the priest arrives at his chair, he leads the assembly in making the sign of the cross, saying: "In the name of the Father, and of the Son, and of the Holy Spirit", to which the people answer: "Amen." Then the priest "signifies the presence of the Lord to the community gathered there by means of the Greeting. By this Greeting and the people’s response, the mystery of the Church gathered together is made manifest" 
Then the priest invites those present to take part in the Act of Penitence and ends it with a prayer of absolution. "From time to time on Sundays, especially in Easter Time, instead of the customary Penitential Act, the blessing and sprinkling of water may take place as a reminder of Baptism."  "After the Penitential Act, the Kyrie, eleison (Lord, have mercy), is always begun, unless it has already been part of the Penitential Act. Since it is a chant by which the faithful acclaim the Lord and implore his mercy, it is usually executed by everyone, that is to say, with the people and the choir or cantor taking part in it." 
"The Gloria in excelsis (Glory to God in the highest) is a most ancient and venerable hymn by which the Church, gathered in the Holy Spirit, glorifies and entreats God the Father and the Lamb... It is sung or said on Sundays outside Advent and Lent, and also on Solemnities and Feasts, and at particular celebrations of a more solemn character."  In accordance with that rule, the Gloria is omitted at funerals. It is also omitted for ordinary feast-days of saints, weekdays, and Votive Masses. It is also optional, in line with the perceived degree of solemnity of the occasion, at Ritual Masses such as those celebrated for Marriage ("Nuptial Mass"), Confirmation or Religious Profession, at Masses on the Anniversary of Marriage or Religious Profession, and at Masses for Various Needs and Occasions.
Liturgy of the Word
On Sundays and solemnities, three Scripture readings are given. On other days there are only two. If there are three readings, the first is from the Old Testament (a term wider than Hebrew Scriptures, since it includes the Deuterocanonical Books), or the Acts of the Apostles during Eastertime. The first reading is followed by a Responsorial Psalm, a complete Psalm or a sizeable portion of one. A cantor, choir or lector leads, and the congregation sings or recites a refrain. The second reading is from the New Testament. The reader typically begins each reading with the introductory statement "a reading from the Book of..." or "A reading from the Letter to...", etcetera (and with the appropriate designation depending on the source), and concludes each reading by proclaiming that the reading is "the word of the Lord," and congregation responds by saying "Thanks be to God." The lector or cantor may be any adult member of the community, and will usually be a scheduled volunteer from among the congregation; when two non-Gospel readings are given, they may be given by two different lectors or by one, according to local preference.
The final reading and high point of the Liturgy of the Word is the proclamation of the Gospel. This is preceded by the singing or recitation of the Gospel Acclamation, typically an Alleluia with a verse of Scripture, which may be omitted if not sung. Alleluia is replaced during Lent by a different acclamation of praise. All stand while the Gospel is chanted or read by a deacon or, if none is available, by a priest. The reading is traditionally introduced with the phrase "a reading from the Holy Gospel according to" followed by the appropriate author's name. To conclude the Gospel reading, the priest or deacon proclaims: "The Gospel of the Lord" and the people respond, "Praise to you, Lord Jesus Christ." The priest or deacon then kisses the book.
If a deacon participates, he reads the Gospel. If a deacon is not present, the celebrating priest or a concelebrant, if there is one, proclaims it.
At least on Sundays and Holy Days of Obligation, a homily, a sermon that draws upon some aspect of the readings or the liturgy of the day, is then given. Ordinarily the priest celebrant himself gives the homily, but he may entrust it to a concelebrating priest or, occasionally, to the deacon, but never to a lay person. In particular cases and for a just cause, a bishop or priest who is present but cannot concelebrate may give the homily. On days other than Sundays and Holy Days of Obligation, the homily, though not obligatory, is recommended.
On Sundays and solemnities, all then profess their Christian faith by reciting or singing the Nicene Creed or, especially from Easter to Pentecost, the Apostles' Creed, which is particularly associated with baptism and is often used in Masses for children.
The Liturgy of the Word concludes with the Universal Prayer or Prayer of the Faithful. The priest begins it with a brief introduction, then a deacon, a cantor or another lay person announces some intentions for prayer, to which the congregation responds with a short invocation such as "Lord hear our prayer". The priest concludes with a longer prayer.
Liturgy of the Eucharist
The linen corporal is spread over the center of the altar, and the Liturgy of the Eucharist begins with the ceremonial placing on it of bread and wine. These may be brought to the altar in a procession, especially if Mass is celebrated with a large congregation. The bread (made only from wheat, recently made) is placed on a paten, and the wine (from grapes), mixed with a little water, is put in a chalice. As the priest places each on the corporal, he says a silent prayer over each individually, which, if this rite is unaccompanied by singing, he is permitted to say aloud, in which case the congregation responds to each prayer with: "Blessed be God forever." Then the priest washes his hands, "a rite in which the desire for interior purification finds expression."
The congregation, which has been seated during this preparatory rite, rises, and the priest gives an exhortation to pray: "Pray, brethren, that my sacrifice and yours may be acceptable to God, the almighty Father." The congregation responds: "May the Lord accept the sacrifice at your hands, for the praise and glory of his name, for our good, and the good of all his holy Church." The priest then pronounces the variable prayer over the gifts that have been set aside.
The Eucharistic Prayer, "the centre and high point of the entire celebration", then begins with a dialogue between priest and people. This dialogue opens with the normal liturgical greeting, "The Lord be with you", but in view of the special solemnity of the rite now beginning, the priest then exhorts the people: "Lift up your hearts." The people respond with: "We lift them up to the Lord." The priest then introduces the great theme of the Eucharist, a word originating in the Greek word for giving thanks: "Let us give thanks to the Lord, our God," he says. The congregation joins in this sentiment, saying: "It is right and just."
The priest then continues with one of many Eucharistic Prayer prefaces, which lead to the Sanctus acclamation: "Holy, Holy, Holy Lord God of hosts. Heaven and earth are full of your glory. Hosanna in the highest. Blessed is he who comes in the name of the Lord. Hosanna in the highest."
The Eucharistic Prayer includes the Epiclesis, through which the Church implores the power of the Holy Spirit that the gifts that have been set aside may become Christ's body and blood and that this Communion may be for their transformation into one body in Christ.
The central part is the Institution Narrative and Consecration, recalling the words and actions of Jesus at his Last Supper, which he told his disciples to do in remembrance of him.
"Take this, all of you, and eat of it: for this is my body which will be given up for you." and "Take this, all of you, and drink from it: for this is the chalice of my blood, the blood of the new and eternal covenant, which will be poured out for you and for many for the forgiveness of sins. Do this in memory of me."
Immediately after the Consecration and the display to the people of the consecrated elements, the priest says: "The mystery of faith", and the people pronounce the acclamation, using one of the three prescribed formulae.
The Eucharistic Prayer also includes the Anamnesis, expressions of offering, and intercessions for the living and dead.
It concludes with a doxology, with the priest lifting up the paten with the host and the deacon (if there is one) the chalice, and the singing or recitation of the Amen by the people. The unofficial term "The Great Amen" is sometimes applied to this Amen.
All together recite or sing the "Lord's Prayer" ("Pater Noster" or "Our Father"). The priest introduces it with a short phrase and follows it up with the embolism: "Deliver us, Lord, we pray, from every evil, graciously grant peace in our days, that, by the help of your mercy, we may be always free from sin and safe from all distress, as we await the blessed hope and the coming of our Savior, Jesus Christ." The people then add the doxology: "For the kingdom, the power, and the glory are yours, now and forever."
Next comes the rite of peace. After praying: "Lord Jesus Christ, who said to your Apostles: Peace I leave you, my peace I give you; look not on our sins, but on the faith of your Church, and graciously grant her peace and unity in accordance with your will. Who live and reign for ever and ever", the priest wishes the people the peace of Christ: "The peace of the Lord be with you always." The deacon or, in his absence, the priest may then invite those present to offer each other the sign of peace. The form of the sign of peace varies according to local custom for a respectful greeting (for instance, a handshake or a bow between strangers, or a kiss/hug between family members).
While the "Lamb of God" ("Agnus Dei" in Latin) litany is sung or recited, the priest breaks the host and places a piece in the main chalice; this is known as the rite of fraction and co-mingling.
If extraordinary ministers of Holy Communion are required, they may come forward at this time, but they are not allowed to go to the altar itself until after the priest has received Communion. The priest then presents the transubstantiated elements to the congregation, saying: "Behold the Lamb of God, behold him who takes away the sins of the world. Blessed are those who are called to the supper of the Lamb." Then all repeat: "Lord, I am not worthy that you should enter under my roof, but only say the word and my soul shall be healed." The priest then receives Communion and, with the help of the deacon and concelebrants and, if necessary, extraordinary ministers of Holy Communion, distributes Communion to the people.
"The faithful communicate either kneeling or standing, as has been determined by the norms of the Conference of Bishops. However, when they communicate standing, it is recommended that before receiving the Sacrament they make an appropriate sign of reverence, to be determined by in the same norms." (In the United States, Communion is to be received standing, though individual members of the faithful may choose to receive while kneeling; and the sign of reverence specified is a bow of the head.)
Then the distributing minister says: "The body of Christ" or "The blood of Christ", according as the element distributed is the consecrated bread or the consecrated wine, or: "The body and blood of Christ", if both are distributed together (by intinction). The communicant responds: "Amen." In most countries the communicant may receive the consecrated host either on the tongue or in the hand, at the communicant's own discretion. In others it may be received only on the tongue. Many faithful then make the sign of the Cross, though this is not prescribed or even recommended in any of the official liturgical texts.
While Communion is distributed, singing by all of an appropriate approved chant or hymn is recommended, to emphasize the essentially "communitarian" nature of the body of Christ. If there is no singing, a short antiphon may be recited either by the faithful, by some of them or by a lector. Otherwise, the priest himself recites it after he himself receives communion and before he distributes it to others.
"The sacred vessels are purified by the Priest, the Deacon, or an instituted acolyte after Communion or after Mass, insofar as possible at the credence table." Then the priest concludes the Liturgy of the Eucharist with the Prayer after Communion, for which the people are invited to stand.
After the Prayer after Communion, announcements may be made. The Missal says these should be brief. The priest then gives the usual liturgical greeting and imparts his blessing. The liturgy concludes with a dialogue between the priest and congregation. The deacon, or in his absence, the priest himself then dismisses the people, choosing one of four formulas, of which the first is "Ite, missa est" in Latin or its equivalent in other languages. The congregation responds: "Thanks be to God." The priest and other ministers then leave, often to the accompaniment of a recessional hymn.
The people then depart. The priest customarily stands outside the church door to greet them individually.
1. John Hardon, Modern Catholic Dictionary
2. Herbermann, Charles, ed. (1913). "Liturgy of the Mass". Catholic Encyclopedia. New York: Robert Appleton Company.
3. Pope Benedict XVI, Sacramentum caritatis, 51
4. "I. THE EUCHARIST - SOURCE AND SUMMIT OF ECCLESIAL LIFE, paragraph 1324". Catechism of the Catholic Church. Retrieved January 19, 2017.
5. "The presence of Christ by the power of his word and the Holy Spirit, paragraph 1373". Catechism of the Catholic Church. Retrieved January 19, 2017.
6. Encyclopaedia Britannica, "Transubstantiation"
7. Catechism of the Catholic Church, 1408
8. "THE ORDER OF MASS" (PDF). International Committee on English in the Liturgy, Inc. 2010. Retrieved January 19, 2017.
9. General Instruction of the Roman Missal (GIRM), paragraph 51
10. General Instruction of the Roman Missal (GIRM), paragraph 52
11. General Instruction of the Roman Missal (GIRM), paragraph 53
12. Roman Missal, "The Order of Mass", 16
13. Roman Missal, "The Order of Mass", 14
14. General Instruction of the Roman Missal (GIRM), paragraph 66
15. "It is a praiseworthy practice for the bread and wine to be presented by the faithful." GIRM, paragraph 73
16. General Instruction of the Roman Missal (GIRM), paragraph 320
17. General Instruction of the Roman Missal (GIRM), paragraph 76
18. General Instruction of the Roman Missal (GIRM), paragraph 78
19. General Instruction of the Roman Missal (GIRM), paragraph 79
20. Luke 22:19; 1 Corinthians 11:24-25
21. General Instruction of the Roman Missal (GIRM), paragraph 151
22. John 14:27
23. General Instruction of the Roman Missal (GIRM), paragraph 162
24. Rev 19:9
25. Matthew 8:8
26. General Instruction of the Roman Missal (GIRM), paragraph 160
27. Matthew 26:26-28
28. General Instruction of the Roman Missal (GIRM), paragraph 287
29. General Instruction of the Roman Missal (GIRM), paragraph 86
30. General Instruction of the Roman Missal (GIRM), paragraph 87
31. General Instruction of the Roman Missal (GIRM), paragraph 279