This trial edition requires Wrath of the Lich King be installed to play, and you have a current subscription or ever have had one to Wrath of the Lich King. This 10-day trial has access to all the content but can't level past 80 (the FAQ specifically states "any characters you play during the trial can only advance up to one experience point below level 81," so if you want to go to Deepholm, Uldum or Twilight Highlands, you'll either need to buy the retail release or, in the case of zones that can be flown to, accept dying a lot.
Seems like an interesting way for the six people in the whole known universe who are on the fence to make up their minds, right? Go read the FAQ and decide if it's for you.
World of Warcraft: Cataclysm has destroyed Azeroth as we know it; nothing is the same! In WoW Insider's Guide to Cataclysm, you can find out everything you need to know about WoW's third expansion, from leveling up a new goblin or worgen to breaking news and strategies on endgame play.