The third of the three sacraments of initiation, the Sacrament of Holy Communion is the reception of Christ's Body and Blood. This sacrament is the source of great graces that sanctify us and help us grow in the likeness of Jesus Christ. Catholics believe the Eucharist, or Communion, is both a sacrifice and a meal. 

The doctrine of the Holy Eucharist consist of that of the Eucharist sacrifice, the sacrificial meal, and the sacrificial food, or to express it otherwise, it consists of the doctrine of the Mass, of Communion, and of the Real Presence. There is no presence of Christ in the Blessed Sacrament that is not meant first and foremost as food for the faithful people, and there is no sacramental union with Christ in Holy Communion that is not to be thought of as a sacrificial meal: 'For as often as you shall eat this bread, and drink the chalice, you shall show the death of the Lord, until he comes' (1 Cor. 11:26).

Catholics believe the Eucharist, or Communion, is both a sacrifice and a meal. We believe in the real presence of Jesus, who died for our sins. As we receive Christ's Body and Blood, we also are nourished spiritually and brought closer to God

From the Catechism of the Catholic Church:

At the heart of the Eucharistic celebration are the bread and wine that, by the words of Christ and the invocation of the Holy Spirit, became Christ’s Body and Blood.  Faithful to the Lord’s command the Church continues to do, in his memory...

 

As early as the second century we have the witness of St. Justin Martyr for the basic lines of the order of the Eucharistic celebration. They have stayed the same until our own day for all the great liturgical families.  St. Justin wrote to the pagan emperor Antoninus Pius (138-161) around the year 155, explaining what Christians did:

On the day we call the day of the sun, all who dwell in the city or country gather in the same place.

The memoirs of the apostles and the writings of the prophets are read, as much as time permits.

When the reader has finished, he who presides over those gathered admonishes and challenges them to imitate these beautiful things.

Then we all rise together and offer prayers* for ourselves... and for all others, wherever they may be, so that we may be, so that we may be found righteous by or life and actions‚ and faithful to the commandments, so as to obtain eternal salvation.

When the prayers are concluded we exchange the kiss.

Then someone brings bread and a cup of water and wine mixed together to him who presides over the brethren.

He takes them and offers praise and glory to the Father of the universe, through the name of the Son and of the Holy Spirit and for a considerable time he gives thanks (in Greek: eucharistian) that we have been judged worthy of these gifts.

When he has concluded the prayers and thanksgivings, all present give voice to an acclamation by saying: ‘Amen.’

When he who presides has given thanks and the people have responded‚ those whom we call deacons give to those present the “eucharisted” bread, wine, and water and take them to those who are absent. - St. Justin, First Apology
 

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