The Catholic distinction grades sin by what it does to the sinner rather than by how the act ranks on a scale of wickedness. Mortal sin is held to destroy charity in the heart and so to end the life of grace in a person; venial sin wounds charity without breaking it. The names carry the claim: mortale from mors, death, and veniale from venia, pardon. The text traditionally cited for the distinction is the first letter of John, which sets sin unto death beside sin that is not unto death.
What makes a particular sin mortal is stated as three conditions that must hold together. The matter must be grave, of the kind the commandments specify. The person must know that it is grave. And the consent must be deliberate. Remove any one and the sin is venial. Ignorance narrows imputability; so does a will that was overwhelmed rather than engaged, by fear, by habit, by compulsion or by the ordinary disorders of a human mind. The tradition draws the obvious conclusion and draws it firmly: no observer can pronounce on another person's state, and very often the person acting cannot pronounce on their own.
Thomas Aquinas supplied the structure behind the rule. On his account mortal sin turns the will away from its last end, while venial sin leaves the end in place and disorders only the means chosen toward it. The two therefore differ in kind and not in degree, which is why the tradition holds that any number of venial sins does not accumulate into a mortal one. They are not small quantities of the same thing.
The remedies follow the diagnosis. Mortal sin is ordinarily remitted through sacramental confession and absolution, and the Council of Trent required that such sins be confessed by kind and by number. Venial sin is remitted by that route too, and also by prayer, by receiving the Eucharist and by acts of charity.