http://tech.fortune.cnn.com/2012/08/04/shot-from-the-air-a-photo-of-the-new-face-of-apples-icloud/?source=yahoo_quote

And what is its purpose?

Some have speculated that it would run iCloud in an emergency should the main data center ever go down. But Apple told county officials it was a "tactical" data center, which in the server business is a term of art.