The greatest of the seven sacraments is the Holy Eucharist. The Catholic Church teaches that in the Eucharist, Our Lord Jesus Christ, true God and true man, is really present under the appearances of bread and wine. Our Lord is not merely symbolized by the bread and wine; nor is he present only through the faith of those present. Rather, the two material things, bread and wine, are completely changed into the body and blood of Jesus Christ, leaving behind only their sensible appearances. Thus, through the words of consecration spoken by the priest, Jesus, without ceasing to be present in a natural way in heaven, is also present sacramentally, body, blood, soul and divinity, in many places throughout the world.

The Eucharist is not only a sacrament but also a sacrifice. In it Jesus, acting through the priest, makes present again in an unbloody manner the sacrifice which he offered once for all by shedding his blood on Calvary. In Holy Communion, by obeying Jesus' command to eat his flesh and drink his blood, the faithful are also united spiritually with Jesus himself, and they unite their own prayers, works and sufferings to his perfect sacrifice.
(EWTN)