From the Catechism of the Catholic Church
1423 It is called the sacrament of conversion because it makes sacramentally present Jesus' call to conversion, the first step in returning to the Father from whom one has strayed by sin.
It is called the sacrament of Penance, since it consecrates the Christian sinner's personal and ecclesial steps of conversion, penance, and satisfaction.
1424 It is called the sacrament of confession, since the disclosure or confession of sins to a priest is an essential element of this sacrament. In a profound sense it is also a "confession" - acknowledgment and praise - of the holiness of God and of his mercy toward sinful man.
It is called the sacrament of forgiveness, since by the priest's sacramental absolution God grants the penitent "pardon and peace."
It is called the sacrament of Reconciliation, because it imparts to the sinner the love of God who reconciles: "Be reconciled to God." He who lives by God's merciful love is ready to respond to the Lord's call: "Go; first be reconciled to your brother."
1431 Interior repentance is a radical reorientation of our whole life, a return, a conversion to God with all our heart, an end of sin, a turning away from evil, with repugnance toward the evil actions we have committed. At the same time it entails the desire and resolution to change one's life, with hope in God's mercy and trust in the help of his grace. This conversion of heart is accompanied by a salutary pain and sadness which the Fathers called animi cruciatus (affliction of spirit) and compunctio cordis (repentance of heart).
There are some acts of the penitent that are important for living the sacrament of confession. Contrition is the pain you feel because of your own sins and includes your decision to not sin again. You should confess grave sins by number and species. (“I masturbated 3 times” or “I missed mass twice”)
After the advice and absolution given by the priest, the penitent is obliged to fulfill the penance. This is called “satisfaction.”