The Holy Eucharist is the greatest of all the sacraments.  It is not merely an instrument for the giving of grace—here is the actual Giver of grace Himself, Jesus Christ our Lord, truly and personally present.

"A giving of thanks"
The sacrament of Christ's Body and Blood has had many names in the course of Christian history.  Such names as Bread of Angels, the Lord's Supper, and Sacrament of the Altar are familiar to us (the Catechism's section on the Eucharist explains several of these common names).   But the name which has endured from the very beginning, the name which the Church officially gives to this sacrament, is that of "Holy Eucharist".  This name is taken from the Bible's accounts of the institution of the Holy Eucharist. They tell us that at the Last Supper, Jesus "gave thanks" as He took the bread and wine into His hands and so from the Greek word eucharistia, which means "a giving of thanks," we have the name of our sacrament: the Holy Eucharist.

Sacrifice and sacrament
The catechism points out that the Holy Eucharist is both a sacrifice and a sacrament.  As a sacrifice the Holy Eucharist is the Mass. The Mass is that divine action in which Jesus, through the agency of the human priest, changes the bread and wine into His own Body and Blood and continues through time the offering which He made to God on Calvary—the  offering of Himself for mankind.

It is at the consecration of the Mass that the sacrament of the Holy Eucharist comes into.  It is then that Jesus becomes present under the appearance of bread and wine. As long as the appearances of bread and wine remain, Jesus remains present and the sacrament of the Holy Eucharist continues to there exist.

The act by which we receive the Holy Eucharist is called Holy Communion.  We might say that the Mass is the "making" of the Holy Eucharist and Holy Communion is the receiving of the Holy Eucharist.  In between the two, the sacrament of the Holy Eucharist continues to exist (as in the tabernacle) whether we receive it or not.

Body and Blood of Christ
St. John's Gospel (Chapter 6) tells us of that day in the town of Capernaum when Jesus made the almost unbelievable promise that He would give His own Flesh and Blood to be the food of our souls.  On the night before he died, Jesus at the Last Supper (and the priest at Mass) said, "This is My Body," over the bread, and "This is My Blood," over the wine.   We believe that the substance of the bread completely and totally ceased to exist, and that the substance of Christ's own Body replaced the substance of the bread. We believe that the wine entirely ceased to exist as wine, and that the substance of Christ's own Blood replaced the wine. This change is called "Transubstantiation".  This miracle requires the right material things to work through, as do all of the sacraments.  The valid matter for the Eucharist is wheat bread and grape wine.  This is the same matter that Jesus himself used at the Last Supper; therefore it is the matter which the priest must use to make Christ's sacramental action present to us in the Mass.

Jesus whole and entire
The Eucharist contains the Real Presence of Jesus: it's the real Jesus—he is truly present, Body and Blood, Soul and Divinity.  This is why such care is taken at Mass in handling not only the consecrated Body and Blood, but also the empty chalice, the sacred  linens used at the altar, and anything else that comes in contact with  the consecrated Eucharistic species.

The Eucharist requires the priesthood
At the Last Supper Jesus changed bread and wine into his own Body and Blood.  At the same time He commanded His apostles to repeat this same sacred action in time to come.  "Do this in remembrance of Me," was the solemn charge which Jesus gave to the Apostles.  Obviously, Jesus does not command the impossible; consequently with this command went also the necessary power, the power to change bread and wine into His Body and Blood.  With the words, "Do this in remembrance of Me," Jesus made His Apostles priests.  This change takes place at Mass when the priest, making himself the free and willing instrument in the hands of Christ, pronounces over the bread and wine Christ's own words, "This is My Body," and "This is My Blood."  Standing at the altar as the visible representative of Jesus and pronouncing Jesus' own words, the human priest "triggers" as it were the infinite power of Jesus, who at that instant becomes present under  the appearances of the bread and wine.

It is in these words—"the words of Consecration" as they are called—that the essence of the Mass resides.

Once the bread and wine have been changed into the Body and Blood of our Lord Jesus, our Savior remains present as long as the appearances of bread and wine remain intact.   In other words Jesus is present in the Holy Eucharist, not just during Mass, but as long as the Sacred Hosts consecrated at Mass continue to retain the appearance of bread.  This means that we owe to the Holy Eucharist the adoration which is due to God, since the Holy Eucharist contains the Son of God Himself.

We adore the Holy Eucharist with the type of worship which may be given only to God.  Every Catholic church has a tabernacle upon the altar.  The tabernacle (from the Latin word tabernaculum, meaning "tent") is a cupboard-like safe which is marked by a burning light called the tabernacle lamp.  Inside the tabernacle, Jesus Christ is present—really and substantially present in the Eucharist.  Christ's presence truly makes our churches the "house of the living God" (1 Tim 3:15).  That is why we maintain a respectful silence while in the church building: to show our respect and reverence of Christ himself.  The presence of Jesus in the tabernacle also makes the tabernacle an excellent place of private prayer.  Even spending just a few minutes sitting quietly in church, contemplating the presence of Christ in the tabernacle or reading from the Gospels, is a commendable practice that greatly aids in spiritual growth.

“For my flesh is true food, and my blood is true drink.  Whoever eats my flesh and drinks my blood remains in me and I in him.” (John 6: 55-56)