This is something that has been rumored for a while.  Some have said it will be an online encyclopedia that J.K. Rowling has been working on since the completion of the last book.  Others thought it would be an MMORPG (Massive Multiplayer Online Role Playing Game) like "World of Warcraft" set at the Harry Potter universe.  Today, J.K. Rowling revealed that Pottermore.com is simply a new experience for Harry Potter fans where they can find new information that didn't make it in the books, buy digital versions and digital audio versions of all seven books, and the site will be "shaped by the readers."  The few who have been given early views of the site have said this is a must for all Harry Potter fans.