Sacraments are celebrations of special moments of encounter between God and human beings. They are not, obviously, the only occasions when human beings consciously respond to God who is forever offering His love and friendship, but they are times when, the Catholic Church teaches, a person's relationship with God is assuredly initiated or deepened or strengthened or healed.
Sacraments are "powers that come 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 (Catechism of the Catholic Church, 1116). The purpose of the sacraments is to sanctify men, to build up the Body of Christ and, finally, to give worship to God. Because they are signs they also instruct. They not only presuppose faith, but by words and objects they also nourish, strengthen and express it (Catechism of the Catholic Church, 1123).
Descriptions courtesy Archdiocese of Toronto