What is the difference between mortal and venial sin?

Scripture itself makes the distinction: "There is sin that is deadly ... and there is sin that is not deadly" (1 Jn 5:16-17). The Church names them mortal and venial.

Mortal sin is a grave violation of God's law that turns us away from Him. It "destroys charity in the heart," and if unrepented at death it means eternal separation from God (CCC 1855-1861). Three conditions must all be met:

1. Grave matter: the act itself is seriously wrong (the Ten Commandments give the outline: murder, adultery, theft of something significant, missing Sunday Mass without cause, and so on).
2. Full knowledge: you knew it was gravely wrong.
3. Deliberate consent: you chose it freely. Fear, coercion, habit, or psychological factors can lessen freedom (CCC 1859-1860).

If any condition is missing, the sin is not mortal, though it may still be serious.

Venial sin is a lesser offense, either in a light matter or in a grave matter without full knowledge or consent. It weakens charity, disposes us to further sin, and merits temporal punishment, but it does not deprive us of sanctifying grace (CCC 1862-1863). Venial sins are forgiven through contrition, prayer, the Eucharist, almsgiving, and the sacramentals.

Why it matters. A person in mortal sin must go to confession before receiving Holy Communion (CCC 1457; 1 Cor 11:27-29). And the Catechism warns against two errors: treating every sin as mortal (scrupulosity) and treating none as mortal (presumption). St. Thomas's image is helpful: venial sin is like an illness; mortal sin is like death. Illness needs care; death needs resurrection, and Christ provides it in the sacrament.