No. 1409 to 1413, pages 355-6 of the Catechism of the Catholic Church, Libreria Editrice Vaticana: Urbi Et Orbi Communications:
The Eucharist is the memorial of Christís Passover, that is, of the work of salvation accomplished by the life, death, and resurrection of Christ, a work made present by the liturgical action.
It is Christ himself, the eternal high priest of the New Covenant who, acting through the ministry of the priests, offers the Eucharistic sacrifice. And it is the same Christ, really present under the species of bread and wine, who is the offering of the Eucharistic sacrifice.
Only validly ordained priests can preside at the Eucharist and consecrate the bread and wine so that they become the Body and Blood of the Lord.
The essential signs of the Eucharistic sacrament are wheat bread and grape wine, on which the blessing of the Holy Spirit is invoked and the priest pronounces the words of consecration spoken by Jesus during the Last Supper: "This is My body which will be given up for you...This is the cup of My blood..."
By the consecration the transubstantiation of the bread and wine into the Body and Blood of Christ is brought about. Under the consecrated species of bread and wine Christ himself, living and glorious, is present in a true, real, and substantial manner: his Body and his Blood, with his soul and divinity (cf: Council of Trent: DS 1640; 1651)
From the Baltimore Catechism No. 3; Benziger Brothers, Inc and Tan Books and Publishers, Inc. No. 895-6, page 191
Q: Why did Christ institute the Holy Eucharist?
A: Christ instituted the Holy Eucharist -
Q: Has the Holy Eucharist any other effect?
A: The Holy Eucharist remits venial sins by disposing us to perform acts of love and contrition. It preserves us from mortal sin by exciting us to greater fervor and strengthening us against temptation.