A MunchKin is a RolePlayer who plays to win, rather than to play, and is generally fairly clueless about anything not explicitly laid out in the rulebooks.
MunchKins treat RPGs as games, not roleplaying.
See:
http://acasun.eckerd.edu/~dagorhir/articles/munchkin
The key quote from this is perhaps "whatever gives the most pluses"!
See also:
A tongue in cheek book about how to be a munchkin: http://www.sjgames.com/munchkin/guide/
A humorous card game about being a munchkin: http://www.sjgames.com/munchkin/game/