Patch 3.1 will represent one of the most significant content patches to World of Warcraft to date. Major modifications to class specializations via the new dual spec feature will bring about significant game play changes for many classes, while the introduction of the Ulduar raid will open up a new and more difficult tier of raiding.
For more information have a look at
http://www.wowinsider.com/guide-to-patch-3-1/