Reforging is a new feature of WoW Cataclysm which allows you to customise your gear by changing some of the stats around.  Reforging is not a profession, it is just a nice new (and expensive) way to change up stats on your gear.

These videos are a preview of the Reforging feature being introduced in World of Warcraft: Cataclysm, as of the most recent beta build.