Sacraments are outward signs instituted by Christ to give inward grace (CCC 1114, 1127). This means that when the sign is performed, such as the pouring of water at baptism or the laying on of hands at confirmation, grace is conferred on the soul. Christ instituted seven sacraments–Baptism, Confirmation, the Eucharist, Reconciliation, Anointing of the Sick, Holy Orders, and Matrimony (CCC 1113).
Baptism is the sacrament through which all sins, actual and original, are forgiven (CCC 1263), and through which we first receive the gift of the Holy Spirit: "Be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit" (Acts 2:38). It is through the waters of baptism that one first receives God's saving sanctifying grace: "Baptism now saves you" (1 Pet. 3:21). Baptism makes one a member of the Catholic Church (CCC 1267), and allows him to receive the other sacraments (CCC 683).
Through confirmation, Christians are anointed with chrism and "sealed with the gift of the Holy Spirit" (CCC 1295), which fully strengthens and equips them to practice and profess their faith openly and without fear (CCC 1303), and results in the full outpouring of the Holy Spirit: "[the Holy Spirit] had not as yet come down upon any of them . . . [Peter and John] upon arriving laid hands on them and they received the Holy Spirit" (Acts 8:16-17).
The Eucharist is the body, blood, soul, and divinity of Christ (CCC 1374), under the appearances of bread and wine: "Jesus took bread . . . he said, ‘This is my body'. Then he took a cup . . . he said, ‘For this is my blood'" (Matt. 26:26-28).
Reconciliation is the means by which post-baptismal sins are forgiven (CCC 1446): "If you [the Apostles, and by extension, all priests] forgive the sins of any, they are forgiven" (John 20:22). "If we confess our sins, [Jesus will] forgive our sins and cleanse us" (1 John 1:9).
The Anointing of the Sick is given to those who are ill. The priests "anoint the sick with oil" (Mark 6:13), a sign of healing (CCC 1293), giving them strength to endure their illness (CCC 1520), and sometimes "restore them to health" (Jas. 5:15). The sacrament remits venial sin, and remits mortal sin when it is combined with sacramental confession, or when the person is unconscious but has made an act of imperfect contrition: "The presbyters [priests] of the Church . . . are to pray over him, anointing him with oil . . . If he has committed any sins, forgiveness will be his" (Jas. 5:14-15).
Through the Sacrament of Matrimony, a man and a woman bind themselves to each other for life by marriage, and "the two . . . become one" (Matt. 19:5).
Through Holy Orders, men receive a share in the priesthood of Christ, and receive the power to confect the sacraments, especially the Eucharist, Reconciliation, Anointing of the Sick, and Confirmation.
The Eucharist, Reconciliation, and Anointing of the Sick can be received more than once. However, Baptism, Confirmation, and Holy Orders can only be received once, because these sacraments leave a "character", or permanent mark on the soul, and receiving the sacrament again would have no effect (CCC 1121). Baptism and Reconciliation are called "sacraments of the dead", because these sacraments bring grace to souls that are devoid of grace, while the Eucharist, Anointing of the Sick , Confirmation, Matrimony, and Holy Orders are called "sacraments of the living", because they are received by people in the state of grace to increase grace in their soul. If the sacraments of the living are received in the state of mortal sin, when there is no grace in the soul, sacrilege is committed (CCC 2120). Furthermore, Baptism, Confirmation, and the Eucharist are called the "Sacraments of Initiation" (CCC 1212), because each of these sacraments integrate Catholics more fully into the life of the Church.
Each sacrament comes with a sacramental grace (CCC 1129). Baptism helps us to reject sin and to have greater faith in God. Confirmation gives us the grace to profess our faith even when it is difficult to do so, and to practice our faith so as to witness to others. The Eucharist helps us to selflessly love God and our neighbor more. Reconciliation strengthens our will against committing sin, helping us to avoid sin in the future. Anointing of the Sick helps us to "bare our crosses". Holy Orders helps priests perform their duties well and make good on their vow. Matrimony gives couples the grace to stay together, and to perform their duties, namely the rearing of their children.
Sacraments are valid if they have valid form, matter, and intention. Form would be the words used and the actions performed. For instance, in baptism, the minister must say, "I baptize you in the name of the Father and of the Son and of the Holy Spirit" (Matt. 28:19), in confession, the priest must say, "I absolve you", and in the Eucharist, the priest must say, "This is my body", and "This is . . . my blood". In baptism, the water must make contact with the skin. Matter is what is used to confect the sacraments. For a valid Eucharist, a certain type of bread and fermented wine must be used, and for a valid Baptism, water must be used. For Confirmation, the Eucharist, Reconciliation, Anointing of the Sick, and Holy Orders, the priest or bishop must confect the Sacrament, whereas in Baptism and Marriage, a lay person, under certain conditions, can perform the sacrament. Intention is necessary, and makes the sacrament valid when the minister of the Sacrament intends to do what the Church does through the sacraments. This does not mean that one necessarily has to believe what the Church believes. After all, many Protestants baptize with the belief that baptism is only a symbol and does not regenerate, but most of these baptisms are recognized as valid. The intention may be implicit, but the intention is still there.