1 John 5:17 talks about two types of sins, "all wrongdoing is sin, but there is sin that is not deadly." In other words, there is sin that is not mortal. A mortal sin destroys your relationship with God and separates you from Him. These are serious sins and will bring about eternal damnation in Hell unless they are repented of.

For a sin to be mortal, it must fulfill three requirements:

1. The sin must be of a serious matter. Some of the less obvious examples of serious sins are contraception, marrying outside the Church (if your Catholic), skipping Mass without a good reason (i.e. if your sick, its ok to stay home), and superstition.
2. The individual must be aware that the sin is serious.
3. There must be a full consent of the free will to perform the sin.

A venial sin is a less serious offence against God. It will not destroy your relationship with God, but it can lead to serious sins and ultimately mortal sins if it persists without repentance.