A sacrament is an outward sign instituted by Christ to give grace. There are seven sacraments: Baptism, Confession, Holy Eucharist, Confirmation, Holy Matrimony, Holy Orders, and Anointing of the Sick.
The sacraments receive their power to give grace from God, through the merits of Jesus Christ.
Presuming the right disposition of the recipient, the sacraments provide the gift of sanctifying grace. They also give a special grace, called sacramental grace, which helps one to carry out the particular purpose of that sacrament.
The chief purpose of Baptism and Confession is to give the supernatural life of sanctifying grace to souls spiritually dead through sin. The chief purpose of Confirmation, Holy Eucharist, Anointing of the Sick, Holy Orders, and Holy Matrimony is to give more grace to souls already spiritually alive through sanctifying grace.
Baptism, Confirmation, and Holy Orders can be received only once because they imprint on the soul a spiritual mark, called a character, which lasts forever.