What happens at Mass?

The shape has not changed since the second century

Around A.D. 155, Saint Justin Martyr described a Sunday gathering: readings from the apostles and prophets, a homily, prayers, the bringing of bread and wine, a long prayer of thanksgiving answered with "Amen," and Communion. That is still the structure of the Mass (CCC 1345).

1. Introductory Rites

Entrance and Sign of the Cross, greeting, the Penitential Act ("I confess…" or "Lord, have mercy"), the Gloria on Sundays outside Advent and Lent, and the Collect (opening prayer).

2. Liturgy of the Word

First Reading, usually from the Old Testament
Responsorial Psalm
Second Reading from the New Testament (Sundays and solemnities)
Gospel Acclamation and the Gospel — all stand
Homily
Creed and Universal Prayer (Prayers of the Faithful)

3. Liturgy of the Eucharist

Preparation of the Gifts — bread and wine are brought to the altar
Eucharistic Prayer — the centre of the Mass. It includes the Sanctus ("Holy, Holy, Holy"), the consecration, when Christ's words are spoken and the bread and wine become his body and blood, and the Great Amen.
Communion Rite — the Our Father, Sign of Peace, Agnus Dei ("Lamb of God"), and Holy Communion

4. Concluding Rites

Blessing and dismissal: "Go forth, the Mass is ended." The word Mass itself likely comes from this sending — missa, "sent."

Why the two parts belong together

On the road to Emmaus, the risen Jesus first explained the Scriptures, and then "was made known to them in the breaking of the bread" (Luke 24:35). Word, then sacrament: every Mass follows that pattern.