A Sacrament is an outward sign instituted by Christ to give grace.The whole liturgical life of the Church revolves around the Eucharistic sacrifice and the sacraments. Sacraments are "powers that comes forth" from the Body of Christ, which is ever-living and life-giving. They are actions of the Holy Spirit at work in his Body, the Church. They are "the masterworks of God" in the new and everlasting covenant.
The Sacraments recall in many ways the means by which Our Lord merited the graces we receive through them. Baptism recalls His profound humility; Confirmation His ceaseless prayer; Holy Eucharist His care of the needy; Penance His mortified life; Anointing of the Sick His model death; Holy Orders His establishment of the priesthood, and Matrimony His close union with the Church.
The sacraments are efficacious ex opere operato (“by the very fact that the sacramental action is performed”) because it is Christ who acts in the sacraments and communicates the grace they signify. The efficacy of the sacraments does not depend upon the personal holiness of the minister. However, the fruits of the sacraments do depend on the dispositions of the one who receives them.
The three sacraments of Baptism, Confirmation, and Holy Orders confer, in addition to grace, a sacramental character or "seal" by which the Christian shares in Christ's priesthood and is made a member of the Church according to different states and functions. This configuration to Christ and to the Church, brought about by the Spirit, is indelible, it remains for ever in the Christian as a positive disposition for grace, a promise and guarantee of divine protection, and as a vocation to divine worship and to the service of the Church. Therefore these sacraments can never be repeated.
Catechism of the Catholic Church - Article 2, The Paschal Mystery in the Church's Sacraments
Compendium of the Catechism of the Catholic Church - Part II, Chapter 1
Baltimore Catechism #3 - Lesson 13