What is a CD key and how do they work on WoW

Basically, in WoW, a CD Key is a code that proves that you have bought the game, and is needed when you set up a WoW account. Once the CD Key is used, it can not be used by any other person. The code is usually found inside the box that the game install disc came in, or in an email if you bought the game from the Blizzard store as a digital download.