In the Old Testament the prophets announced that the Spirit of the Lord would rest on the hoped-for Messiah for his saving mission. The descent of the Holy Spirit on Jesus at his baptism by John was the sign that this was he who was to come, the Messiah, the Son of God. He was conceived of the Holy Spirit; his whole life and his whole mission are carried out in total communion with the Holy Spirit whom the Father gives him "without measure."

This fullness of the Spirit was not to remain uniquely the Messiah's, but was to be communicated to the whole messianic people. On several occasions Christ promised this outpouring of the Spirit, a promise which he fulfilled first on Easter Sunday and then more strikingly at Pentecost. Filled with the Holy Spirit the apostles began to proclaim "the mighty works of God," and Peter declared this outpouring of the Spirit to be the sign of the messianic age. Those who believed in the apostolic preaching and were baptized received the gift of the Holy Spirit in their turn. 

It is evident from its celebration that the effect of the sacrament of Confirmation is the special outpouring of the Holy Spirit as once granted to the apostles on the day of Pentecost. From this fact, Confirmation brings an increase and deepening of baptismal grace: 

it roots us more deeply in the divine filiation which makes us cry, "Abba! Father!" 

it unites us more firmly to Christ; it increases the gifts of the Holy Spirit in us;

it renders our bond with the Church more perfect;

it gives us a special strength of the Holy Spirit to spread and defend the faith by word and action as true witnesses of Christ, to confess the name of Christ boldly, and never to be ashamed of the Cross. 

Like Baptism which it completes, Confirmation is given only once, for it too imprints on the soul an indelible spiritual mark, the "character," which is the sign that Jesus Christ has marked a Christian with the seal of his Spirit by clothing him with power from on high so that he may be his witness. 

"Now when the apostles at Jerusalem heard that Samaria had received the word of God, they sent to them Peter and John, who came down and prayed for them that they might receive the Holy Spirit; for it had not yet fallen on any of them, but they had only been baptized in the name of the Lord Jesus. Then they laid their hands on them and they received the Holy Spirit" (Acts 8:14-17). 

Confirmation perfects Baptismal grace; it is the sacrament which gives the Holy Spirit in order to root us more deeply in the divine filiation, incorporate us more firmly into Christ, strengthen our bond with the Church, associate us more closely with her mission, and help us bear witness to the Christian faith in words accompanied by deeds. 

A candidate for Confirmation who has attained the age of reason must profess the faith, be in the state of grace, have the intention of receiving the sacrament, and be prepared to assume the role of disciple and witness to Christ, both within the ecclesial community and in temporal affairs. 

The essential rite of Confirmation is anointing the forehead of the baptized with sacred chrism, together with the laying on of the minister's hand and the words: Be sealed with the Gift of the Holy Spirit in the Roman rite. 

When Confirmation is celebrated separately from Baptism, its connection with Baptism is expressed, among other ways, by the renewal of baptismal promises. The celebration of Confirmation during the Eucharist helps underline the unity of the sacraments of Christian initiation.

This information taken from Catechism of the Catholic Church #1285 - 1321