WoW World of Warcraft Demo
A Demo version of a game is a limited version that allows you to test the game before you buy it. World of Warcraft will probably not have a demo version because it is not practical for a MMORPG.
Instead of a Demo, World of Warcraft will have a free Trial Version, to learn more on this subject read more about the Free Trial. It is unsure whether or not a Free Trial will be available from at the time of release.