Mortal sin is called mortal because it is the "spiritual" death of the soul (separation from God). If we are in the state of grace it loses this supernatural life for us. If we die without repenting we will lose Him for eternity. However, by turning our hearts back to Him and receiving the Sacrament of Penance we are restored to His friendship. Catholics are not allowed to receive Communion if they have unconfessed mortal sins.
Venial sins are slight sins. They do not break our friendship with God, although they injure it. They involve disobedience of the law of God in slight (venial) matters. If we gossip and destroy a person's reputation it would be a mortal sin. However, normally gossip is about trivial matters and only venially sinful. Additionally, something that is otherwise a mortal sin (e.g. slander) may be in a particular case only a venial sin. The person may have acted without reflection or under force of habit. Thus, not fully intending the action their guilt before God is reduced. It is always good to remember, especially those who are trying to be faithful but sometimes fall, that for mortal sin it must not only be 1) serious matter, but 2) the person must know it is serious and then 3) freely commit it.