Well Croz,
Halloween King could give you a better and more eloquent answer, him bein' a gothic scolar an' all, but basically, a Doppelganger is a 'double' of someone, usually malicious, and often an insane alter ego of the person in in question.
The Doppelganger is a favorite trope of gothic and Victorian literature, appearing in the works of the Shelleys, (both Mary and Percy), Edgar Allen Poe (his story 'William Wilson' being one example), and Robert Louis Stevenson's 'Strange Case of Doctor Jekyll and Mr. Hyde.'
Some of you may find that the part of yourselves that so enjoys hotrodding is a sort of doppelganger to the part of you that is a good student, a responsible parent & husband, etc. <img src="graemlins/pimp.gif" border="0" alt="[pimp]" />