A venial sin meets at least one of the following criteria:
Each venial sin that one commits adds to the penance that one must do. Penance left undone during life converts to punishment in purgatory. A venial sin can be left unconfessed, though (so long as there is some purpose of amendment) is helpful to confess, for one receives from the sacrament grace to help overcome it.
Venial sins usually remain venial no matter how many one commits. They cannot "add up" to collectively constitute a mortal sin. There are however some cases were repeat offenses may become grave matter. For instance if one were to steal small amounts of property from a single person, over time one would have stolen enough that it would become a serious theft from that person.
In all this one ought not to take venial sin, especially deliberate, lightly. No one without a special grace (according to most theologians given only to the Blessed Virgin Mary) can avoid even semi-deliberate venial sins entirely (according to the definition of Trent). But one must, even to avoid mortal sins, seek as far as posible to overcome venial sin, for though a number of venial sins do not themselves add up to a mortal sin, each venial sin weakens the will further, and the more willing one becomes in alowing such falls, the more one is inclined towards, and will inevitably fall into (if one continues along this path) mortal sin.
In 1 John, the author distinguishes between two types of sin (1 John 5:16-17). According to a Roman Catholic commentary on the Bible, however, these verses do not refer precisely to venial and mortal sins. One class of sin leads to the loss of eternal life, but eternal life can be regained if a fellow Christian prays that the sinner be forgiven. The other class of sin leads to death, and the Christian is not instructed to pray that a fellow Christian be forgiven for such a sin. Since either class of sin puts one's eternal life in jeopardy, both would fall in the category of mortal sin.