The Catechism describes two main types of venial sin. First, one commits venial sin when "in a less serious matter [than mortal sin], he does not observe the standard prescribed by the moral law" (CCC 1862). In other words, if one does something immoral but the matter is not serious enough to be gravely immoral, he commits only venial sin.
For example, deliberate hatred can be venial sin or mortal sin depending on the seriousness of the hatred. The Catechism explains, "Hatred of the neighbor is a sin when one deliberately wishes him evil. Hatred of the neighbor is a grave sin when one deliberately desires him grave harm" (CCC 2303).
Another example is abusive language. "Abusive language is forbidden by the fifth commandment but would be a grave offense only as a result of circumstances or the offender’s intention" (CCC 2073).
The second type of venial sin involves situations in which the matter is serious enough to be gravely immoral, but the offense lacks at least one of the other essential elements required for mortal sin. The Catechism explains that one commits only venial sin "when he disobeys the moral law in a grave matter but without full knowledge or without complete consent" (CCC 1862).
An example of this could be masturbation. The Catechism explains:
To form an equitable judgment about the subjects’ moral responsibility [for masturbation] . . . one must take into account the affective immaturity, force of acquired habit, conditions of anxiety, or other psychological or social factors that can lessen, if not even reduce to a minimum, moral culpability. (CCC 2352)