A mortal sin is the most serious type of sin in Christianity. Types of mortal sin include idolatry, adultery, murder and slander. These sins are more serious than venial sins because they go against the most basic teachings of Christianity,
Serious sins are called "mortal" because Christians believe that these sins cause the spiritual death of the soul. One must repent after committing one of these sins in order to be admitted into Heaven after death. If the sinner does not repent before death, then it is believed that God will no longer accept that person into Heaven.
Venial sins are those which have less of an impact on the world and are thus considered to be less serious. The sinner must repent for these sins as well, but committing them does not exclude one from reaching Heaven after death.
According to Christian doctrine, a mortal sin is only considered such if the sinner knows the seriousness of the sin and commits it anyway. Therefore, someone who commits slander thinking it was just harmless gossip would not be considered a mortal sinner, only a venial sinner. A confession and repentance is still necessary, but not to the same degree as for a mortal sin.