WoW Blog

A key is an item that can be used to unlock a specific or general type of locked container in WoW. Blacksmiths can create skelton keys and use them, usually souldbound them (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 Keys example: The Key equired 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.