Plasma guns fire balls of superheated gas known as plasma.  The explosion of a plasma impact is one of the most potent weapons of the  40K battlefield.  Plasma weapons have a repuation for unreliablility, so only the most experinced warriors are equipped with plasma weapons. 
Due to it's rareity and requiring especialy skilled users plasma guns are usualy only given to Terminator elites.
Usually used in combination with a Power Fist.
