basically, each tree represents a different role in the game. arms is straight up PVP, but can be used for leveling quite quickly if played well. Fury is the best leveling tree and later on if you get good enough with it you can easily become a major DPS in raids and instances. Prot though, is for tanking, and... you can PVP with it. Basically Prot warriors don't die so they just build up rage and execute the mess out of people. If you don't believe me, try it for yourself.

I'd recommend a hybrid though. Whatever build you take, putting some points into Cruelty (+1% to crit chance) is never a bad idea. Neither is Anticipation (+2 to defensive rating).

And, funny story: I'd recommend dual-wielding as an Arms warrior. Arms warriors DPS is based off of crits and bleeding. There's points which make your opponent bleed for every crit, increase damage of your rend, increase damage of your crit, increased damage while bleeding, up to a 50% increase of an overpower crit, and the lovely mortal strike which is the greatest thing ever. great crit chance (1 bleed) and makes them bleed after its successful (2 bleeding effects from one hit).