[Rem]

This is what J.K. Rowling says about it:
Pottermore is a unique online reading experience created by J.K. Rowling and built around the Harry Potter books. Share and participate in the stories, showcase your own Potter-related creativity, and discover additional information about the world of Harry Potter from the author herself

It's basically a website that has a bunch of interactive stuff regarding the books, and I'm pretty sure some extra never-before seen stuff. And a site for HP fans to get together and do things, like games and chat and showcase their art and etc.