There's a huge back story to the WOW game. You can read some excerpts from various Blizzard novels here.
Here is the Warcraft Encyclopedia.
WOW Insider has a weekly column called Know Your Lore. Head to their website and search on "know your lore" to see the columns to date.
There is a website devoted to WOW lore. Sadly, it doesn't seem to have much up but perhaps it will start being populated by more than forums, photos and news tidbits soon.
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