Sacrilege is the irreverent treatment of sacred things, people or things.
A mortal sin is so called because it is considered to be the death of the soul. It applies to such things as murder and adultery, if carried out with full knowledge and full consent. Catholics may not receive Communion if they have not confessed to mortal sins.
Venial sin is not so serious – for example, selfishness, anger, or lack of generosity.