"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 being (or is “confected” as the theologians say). 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.

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. It is marked by a burning light called the tabernacle lamp. Following an ancient tradition, it is often covered with a veil as an indication of the holiness of the place.

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."

  -The Eucharist: In the Presence of the Lord Himself, Retrieved from http://www.beginningcatholic.com/eucharist.html