Key
There are a few types of keys:
- An item that can be used to unlock a specific or general type of locked container. Blacksmiths can create skeleton keys and use them (skeleton keys require the same blacksmithing skill to use as to create). These are destroyed upon use.
- A usually soulbound item (often a quest reward) that can be used to unlock a specific or doors and such (gates, hatches, etc.). For example,
[The Scarlet Key] will open the door to Scarlet Cathedral and Scarlet Armory within the Scarlet Monastery and the Scarlet Bastion within Stratholme. The 
[Key to Searing Gorge] will open the gate into the Searing Gorge from Loch Modan. These keys are not destroyed upon use. - Quest item "keys": See below for explanation.
Quest item keys
One example of a quest item key would be the 
[Defias Gunpowder] required for the cannon near the end of the Deadmines. It is picked up and consumed each dungeon run. It is not exactly a quest item, but it is the only way to open the cannon, it is consumed there, and it cannot be used anywhere else.
Another example would be the 
[Drakefire Amulet], commonly referred to as the "Onyxia Key" or "Ony Key". It was required to gain access to Onyxia's Lair. The method to obtain the amulet varies between the Alliance and the Horde. See [Drakefire Amulet] for details on how to obtain it.
Keyring
Patch v1.11 added a "bag" called a keyring. It is an expandable bag that can store various types of keys, reducing the number of backpack and vault slots filled with these re-usable items. Dungeon keys (like the Scarlet Key (Classic)) and Blacksmithing keys (such as 
[Titanium Skeleton Key]) may also be stored in the keyring.
Lock Picking
A rogue with enough skill in the ability 
[Pick Lock] can open most of the locked containers or doors that require keys, with the exception of a few high level dungeon doors.
In Cataclysm
In Cataclysm most keys associated with instances have be removed from the game and the doors they opened no longer need something (including lock picking) to allow access. In light of this, 
[The Keymaster] achievement was removed entirely.
The Keymaster
[The Keymaster] was a General Achievement to collect many of the dungeon related keys in the game, spanning from World of Warcraft and all the way up to World of Warcraft: Wrath of the Lich King. The following were required for the Achievement (See The Achievement Page for Information on obtaining each key):
- [Auchenai Key]
[Key of Time]
[Shadow Labyrinth Key]- [Shattered Halls Key]
- [Skeleton Key]
- [The Violet Hold Key]
- [Key to the City]
[Shadowforge Key]- [The Scarlet Key]
[Workshop Key]