"Rules lawyers are a common breed of gamer that basically exist to make sure that every single game you ever wanted to play takes four times as long. You want to just take the hit, make your fortitude save, and move on? Too bad, the rules lawyer is arguing with the DM about how many people can physically fit in a single five-foot-square, just so his character doesn't have to take d6 damage from the poison arrow trap. You want to assault your opponent's Fire Warriors with your leaping Hormagaunts? Well boo for you, you moved that one model too far during the movement phase; you need to move it back until it conveniently no longer has charge range. The rules lawyer is a vampire, sucking all the fun out of the game, and the sooner you drive a stake through his withered husk, decapitate and bury his body, and never speak of him again, the happier everyone will be."
Rules lawyers are usually That Guy at the table, who is gonna get butthurt when you handwave a mechanic that is essential to their min/max munchkin build. They truly believe that Rule Zero is a blasphemy, and can't act in-character for shit. They'd rather roll Diplomacy checks than negotiate with an NPC at the table. They don't care whether anyone else at the table is having fun, they don't care whether their nitpicky, fussy, little corrections actually matter to the outcome of the game; and whenever they open their stupid gobs and thick books all the other players groan and know that things are going to come to a crashing halt, and that what little time they have to enjoy their hobby each week is about to get sucked into some sort of gigantic cosmic black-ass-hole.
Some DMs ask their rules lawyers to keep everyone (including the DM) honest; after all, the guy who has all the books memorized will probably be able to tell if one of the players is pulling a fast one, or help the DM figure out how to rule something unexpected. The best of these will even flatly tell the DM "no, I didn't escape the deathtrap, I died".
Like all lawyers, rules lawyers end up in hell where they are assigned to make sure Faustian pacts don't have loopholes that can be exploited by the pact-ee.
- Munchkin - If the would-be rules lawyer is also a hypocritical little bitch who conveniently only complains when the ruling would hurt him personally, and becomes curiously silent when it would work in his favor, he's probably this instead. Same poison, higher dose.