A type of ranged weapon that usually causes only physical damage and doesn't need ammo to use, instead the weapon loses 1 out of 200 durability per throw, and requires a repair once it reaches 0 durability.
All Rogues start with the throw ability, and later gain the ability Deadly Throw, which requires a thrown weapon to be equipped. Hunters and Warriors can learn the throw skill from some trainers (see weapon trainers for details).
'Throwing' is not the same as 'shooting'. In particular, hunter skills that work with shooting (all of the ranged hunter skills?) do not work with a thrown weapon. So while a hunter can use a thrown weapon, it is a poor choice for a hunter.
Warriors and rogues will not be relying on ranged attacks for damage. They will sometimes use a ranged attack to call a mob out of an area into melee to isolate it from other mobs. Warriors and rogues lack the skills that make shooting so preferable to a hunter, so for them, throwing is a good option. For them, a thrown weapon is a ranged weapon with intrinsic ammo, and they will not burn through the 'ammo' nearly as fast as a hunter would.
Drysc: "The mechanic of thrown weapons is being changed in the expansion to be similar to a normal weapon. Once a stack is used it will remain in your inventory and can then be repaired to refill the stack, similar to repairing broken armor or weapons. This allows us to put stats on thrown weapons and itemize them as we would any other weapon." 
Some examples : http://www.thottbot.net/beta?iclass=2.16
There is a bug currently in game that still causes throwing weapons to be used up even though the patch was implemented to stop this. Blizzard claims they are working on a fix for this bug.