The Mass (the Eucharist) is “the source and summit of the Church’s life”. In it, Christ is really, truly and substantially present under the appearances of bread and wine. Catholics listen to the Word of God and receive Christ’s Body and Blood, miraculously transformed from bread and wine, at Holy Communion.
The Fathers of the Second Vatican Council describe the Mass in the following way:
At the Last Supper, on the night when He was betrayed, our Saviour instituted the eucharistic sacrifice of His Body and Blood. He did this in order to perpetuate the sacrifice of the Cross throughout the centuries until He should come again, and so to entrust to His beloved spouse, the Church, a memorial of His death and resurrection: a sacrament of love, a sign of unity, a bond of charity, a paschal banquet in which Christ is eaten, the mind is filled with grace, and a pledge of future glory is given to us (Sacrosanctum Concilium 47).
At Mass, the sacrifice of the Cross is renewed in an “unbloody manner”, as the memorial of Christ’s death.
The liturgy of the Mass is itself divided into two parts. The first part is the liturgy of the Word, in which we listen to Christ speaking in the Scriptures. The second part is the liturgy of the Eucharist, where the sacrifice of Calvary is renewed. Strictly speaking, however, the entire Mass is one sacrifice. That is why it is sometimes called the “Sacrifice of the Mass”.