"According to St. Thomas, in order to live spiritually man must remain in communion with the supreme principle of life, which is God, since God is the ultimate end of man’ s being and acting. Now sin is a disorder perpetrated by the human being against this life-principle. And when through sin, the soul commits a disorder that reaches the point of turning away form its ultimate end God to which it is bound by charity, then the sin is mortal; on the other hand, whenever the disorder does not reach the point of a turning away from God, the sin is venial."
RECONCILIATION AND PENANCE, JOHN PAUL II, n. 17
Mortal sins deserve eternal punishment, if the individual never repents; venial sins are not deserving of eternal punishment. Mortal sins are in a matter serious enough to end one’s friendship with God.
As for mortal sins of omission, these would have to violate the positive precepts, i.e. commandments that require us to act, rather than to refrain from acting. Examples:
Worship God, keep holy the Sabbath, honor your father and mother, love your neighbor, prayer, self-denial (especially during lent), works of mercy
One is not required to fulfill positive precepts continually, whereas one must continually avoid violating negative precepts. While the Church currently requires attendance at Mass on Sundays and holy days of obligation, the Church could change that requirement; the obligation under Divine Law to worship God and keep the Sabbath holy can be fulfilled in many ways.
All the positive and negative precepts can be summed up by two rules:
love your neighbor.