A player who tweaks their character in order for it to have the most, best, coolest abilities. They then proceed to play in a fashion contrary to most peoples' enjoyment of a game by constantly trying to amass the most kills or experience points or some other goal, usually at the expense of most other characters and players. They often do this in the hopes of "winning" the game, although most players agree that there is often no winning in a roleplaying game. The goal is to have fun and create an engaging story. Obviously, the bane of many games and Storytellers.
The term "twink" carries a certain connotation, referring to someone who abuses online RPGs in this manner, such as a MUSH or MMORPG. For standard pen and paper-style games, it is more common to use the term munchkin.