Reward

One or more items, abilities, spells, or money given by a quest giver NPC for completing a quest. All quests tend to include XP as part of the reward.

Additional notes:
 * At lower levels, most rewards are items and/or money.
 * Sometimes a reward or part of the reward is another quest.
 * Some quests allow you to choose among reward items.
 * Most reward items will be bind on pickup, also known as Soulbound.
 * The completion of quests will also impact your reputation with particular factions, which can have further beneficial effects such as cheaper prices at vendors and access to special vendors such as mounts.
 * Some reward items may not be usable by your character (based on class usually).
 * At level 80 all quest rewards are money, based on XP usually gained from the quest

Mists of Pandaria
In Mists of Pandaria (specifically, the Wandering Isle and Pandaria zones), quest rewards go from a pre-set of items, often times not useful to many classes, to be more dynamic. When completing a quest, the player only sees items that are made for them, out of the dozen or so possibilities. This does however mean if the reward is not something you want (an upgrade) you won't be able to pick the item worth the most gold in order to sell to a vendor as players were able to do prior to this expansion.

The dynamic rewards take both class and specialization into account. For example, protection warriors see dodge/parry plate, while arms and fury warriors see crit/haste plate. Thus, sometimes it is possible to see a different reward by activating a different spec.