A sacrament is an outward sign instituted by Christ to give grace.
There are seven sacraments: Baptism, Confirmation, Holy Eucharist, Penance, Anointing of the Sick, Holy Orders and Holy Matrimony.
From the Catechism of the Catholic Church
1210 Christ instituted the sacraments of the new law. There are seven: Baptism, Confirmation (or Chrismation), the Eucharist, Penance, the Anointing of the Sick, Holy Orders and Matrimony. The seven sacraments touch all the stages and all the important moments of Christian life: they give birth and increase, healing and mission to the Christian's life of faith. There is thus a certain resemblance between the stages of natural life and the stages of the spiritual life.
1211 Following this analogy, the first chapter will expound the three sacraments of Christian initiation; the second, the sacraments of healing; and the third, the sacraments at the service of communion and the mission of the faithful. This order, while not the only one possible, does allow one to see that the sacraments form an organic whole in which each particular sacrament has its own vital place. In this organic whole, the Eucharist occupies a unique place as the "Sacrament of sacraments": "all the other sacraments are ordered to it as to their end."2
1212 The sacraments of Christian initiation - Baptism, Confirmation, and the Eucharist - lay the foundations of every Christian life. "The sharing in the divine nature given to men through the grace of Christ bears a certain likeness to the origin, development, and nourishing of natural life. The faithful are born anew by Baptism, strengthened by the sacrament of Confirmation, and receive in the Eucharist the food of eternal life. By means of these sacraments of Christian initiation, they thus receive in increasing measure the treasures of the divine life and advance toward the perfection of charity."
2St. Thomas Aquinas, STh III, 62,3