Rules Lawyer

Objection! I was not in range to take that d6 of damage!

"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."

- Faggot McStoner after the 11th time he has been called out for not knowing the rules to the game the group is playing/Angry Gamer with a legitimate grievance against That Guy

A Wargamer speaks

Rules Lawyers are there to ensure everyone is actually playing the game properly and not inadvertently cheating. In competitive games this is more important than cooperative games. In a game of 40k, letting a unit arbitrarily make an assault when it is not entitled to is cheating and makes any victory that results in it invalid, and anyone who takes pleasure from it a massive bitch.

A Roleplayer speaks

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.

Both sides have some validity, although it is worth pointing out that there is a fair distance between someone who carefully watches his opponents dice rolls and measurements to make sure that he's playing fair and someone who constantly comes up with broken situations that abuse the mechanics in order to win. In competitive games, the rules-lawyer kind of player is likely to insist on the rules as written are all that is important, and that's why he can do his retarded tactic that wins on turn 2 eeeeeeverysingle game and then refuses to come back when the group starts a houserule of 'No doing that shit'. In co-operative games, the rules-lawyer is likely to insist that his interpretation is right regardless of the GM applying physical violence and telling him to STFU, and is also likely to own every single book ever published for a system and constantly insist that they all be used even when its inpractical to actually carry them around.

Oh and a rules lawyer may not actually be a munchkin or min/maxer, although they do often come together. They just want the rules to favor them and their plans so they walk through the universe a charmed individual who is wearing rules proof armor.