Optimizer: Some one who picks a character concept and then makes the most mechanically viable character that fulfills said concept.

Powergamer: Someone who chooses a specific power level that they want their character to be and then builds a character that fulfills this criteria. After that is done they come up with the concept and background of the character based on it's mechanics.

Munchkin: Someone who makes the most powerful character they can get away with, not necessarily with any regard for the actual rules.

Both optimizers and powergamers are relatively good things and have their uses.

Munchkins are ***** and should be booted from the group immediately because they will just cause problems.