Actual sin is the sin human beings commit once they have reached the age of reason (seven years old).
It is an intentional, deliberate, conscious, and willful act to oppose the will of God. Sin is more than just breaking the divine law; it is a spiritual disease that harms the life of grace.
Some sins are so dangerous that they can kill the life of grace and permanently destroy our relationship with God for all eternity. These deadly sins are called mortal sins. Venial sin is not as deadly.
Some Catholics try to understand sin through a legal metaphor, as though mortal sins are felonies and venial sins are misdemeanors. While this analogy may help, it has its limitations. It is better to think of mortal sin as a malignant tumor which, if left untreated (no confession), will eventually kill the spiritual life of the person. Venial sin in this model is a benign tumor, although not a life-threatening condition.
Sanctifying grace heals original sin, and actual grace heals actual sin. Sanctifying grace comes only through the sacraments, first and foremost Baptism and Penance (Confession). Actual grace comes just by asking for it, by reading the Bible, using a sacramental, or by God’s divine mercy.
Actual grace helps us resist temptation and empowers a person both to want to do and to do good works. Without this grace, man cannot do any holy deed. He can do good and virtuous deeds, but not easily. He cannot do any supernatural work without divine grace.
Actual sin can keep us from heaven if serious enough, but even minor sins over time will weaken and discourage the most dedicated person.