These weapons, except for wands, are equippable by hunters, rogues, and warriors in their ranged weapon slot. Wands can be used by mages, priests, and warlocks in their ranged weapon slot. No ranged weapon requires ammo, and they do not need any sort of recharge to continue working.
Except for wands, all ranged weapons have a short-range limitation in that they cannot be used on enemies in melee range.
Bows, guns, and crossbows require the Shoot ability, whereas thrown weapons require the Thrown ability. These two abilities are not interchangeable even though the ranged slot can accommodate both equally well. This means that if you equip a thrown weapon after having had a shooting weapon, you will have to switch to the Thrown ability (often bound to an Action Bar), instead of the Shoot ability.
Mentioned in contrast to melee weapons.
In Mists of Pandaria
This section concerns content exclusive to Mists of Pandaria.
In Mists of Pandaria, the separate Ranged/Relic equipment slot was removed, so classes that could use ranged weapons could no longer have both a melee and ranged weapon equipped. The minimum range limitation was removed and all ranged weapons now counted as two-handed weapons.
This section concerns content exclusive to Cataclysm.
In Catclysm, the need for separate ammo for ranged weapons was removed.