The Sacrament of Baptism is often called "The door of the Church," because it is the first of the seven Sacraments not only in time (since most Catholics receive it as infants) but in priority, since the reception of the other sacraments depends on it. It is the first of the three Sacraments of Initiation, the other two being the Sacrament of Confirmation and the Sacrament of Holy Communion. Once Baptized, a person becomes a member of the Church. Traditionally, the rite (or ceremony) of Baptism was held outside the doors of the main part of the church, to signify this fact.
In the Catholic Church today, Baptism is most commonly administered to infants. While some other Christians strenuously object to infant Baptism, believing that baptism requires assent on the part of the person being baptized, the Eastern Orthodox, Anglicans, Lutherans, and other mainline Protestants also practice infant Baptism, and there is evidence that it was practiced from the earliest days of the Church.
Since Baptism removes both the guilt and the punishment due to Original Sin, delaying Baptism until a child can understand the sacrament may put the child's salvation in danger, should he die unbaptized.
Adult converts to Catholicism also receive the Sacrament, unless they have already received a Christian Baptism. (If there is any doubt about whether an adult has already been Baptized, the priest will perform a conditional Baptism.) A person can only be Baptized once as a Christian—if, say, he was Baptized as a Lutheran, he cannot be rebaptized when he converts to Catholicism.
While an adult can be Baptized after proper instruction in the Faith, adult Baptism normally occurs today as part of the Rite of Christian Initiation for Adults (RCIA) and is immediately followed by Confirmation and Communion.
Baptism has six primary effects, which are all Supernatural Graces:
The removal of the guilt of both Original Sin (the sin imparted to all mankind by the Fall of Adam and Eve in the Garden of Eden) and personal sin (the sins that we have committed ourselves).
The remission of all punishment that we owe because of sin, both temporal (in this world and in Purgatory) and eternal (the punishment that we would suffer in hell).
The infusion of Grace in the form of Sanctifying Grace (the life of God within us); the Seven Gifts of the Holy Spirit; and the three Theological Virtues.
Becoming a part of Christ.
Becoming a part of the Church, which is the Mystical Body of Christ on earth.
Enabling participation in the Sacraments, the priesthood of all believers, and the growth in Grace.
Information taken from About Religion website: http://catholicism.about.com/od/beliefsteachings/p/Sac_Baptism.htm