A sacrament is "an outward sign, instituted by Christ, to give grace." Let's look at each of the three elements.
"Outward sign" means that there is a visible action that takes place. In Baptism, for example, it is the pouring of water on the head of the baptized, accompanied by the words "I baptize you..." In Confirmation, the presider lays his hands on the individual being confirmed and says the appropriate prayer.
"Instituted by Christ" means that during his life on earth, Jesus created each sacrament by something he did or said. For example, Jesus' baptism in the Jordan, or His granting of authority to the Apostles to forgive sins.
"To give grace" means that the very act calls forth a special grace from God for the person receiving the sacrament.
There are seven sacraments. Each has its own page at the right.