The Church draws her life from the Eucharist. In the Holy Eucharist, through the changing of the bread and wine into the Body and Blood of the Lord, she rejoices in this presence with unique intensity. (St. John Paul II, On the Eucharist [Ecclesia de Eucharistia, EE] no.1)
At Mass, the consecrated bread is Christ's body, the consecrated wine is Christ's blood. He is fully present under each form of the Eucharist. This happens by the power of the Holy Spirit through the ministry of the priest's acting in the person of Christ during the Eucharistic prayer. -United States Catholic Catechism for Adults