From the Leaky Cauldron, who have a transcript of the press conference:
Pottermore, it was revealed, is “a unique and free-to-use website which builds an exciting online experience around the reading” of the Harry Potter books. The author has written thousands of words of new material “about the characters, places and objects in the much-loved stories, which will inform, inspire and entertain readers as they journey through the storylines of the books. Pottermore will later incorporate an online shop where people can purchase exclusively the long-awaited Harry Potter eBooks, in partnership with J.K. Rowling’s publishers worldwide, and is ultimately intended to become an online reading experience, extending the relevance of Harry Potter to new generations of readers, while still appealing to existing fans.”
The e-books will not, as some feared, be Sony-exclusive – they will be compatible with all e-formats. The site will officially open in October with a rumoured 18,000 words from Rowling adding to background stories and wizarding history thus far only alluded to in the books, with much buzz about new material on Minerva MacGonagall.
From her press conference:
“The other thing is, this is free. You don’t have to buy anything to access what you’ve seen today. You don’t have to pay to get the extra material, you don’t have to buy a single thing to go onto Pottermore and have the whole experience you’ve just seen. That was really important to me. This was about the give-back. The technology now existed to do something outside the books and the films for existing fans.”