A term used by my friends and I to describe certain game players (typically paintball players). A shark will spare little to no expense to give himself an incredible advantage over other players.

For example, in paintball, sharks are those players who purchase guns with a base price of $500+, and then spend anywhere from $200-$2000 more on said gun. On a small, regulated field where most people play, they have an overwhelming advantage in shot velocity, accuracy, and rate of fire, especially against those people armed with rental guns. Since I am a poor college student, I cannot afford to spend the amount of money it would require to put up a decent challenge to these people, even though I have the necessary combat skills.
This definition is not limited to paintball, but can also be applied, for example, to those Quake players who spend as much money as possible outfiting their computers with the latest in hardware in order to give themselves an advantage in frame rate.

Sharks take the fun out of most games.