According to the article:
Waterstone’s Booksellers Ltd. says the cream-colored A5 papers — each slightly bigger than a postcard — were distributed to 13 authors and illustrators, including the boy wizard’s creator J.K. Rowling, Nobel Prize winner Doris Lessing, novelist Margaret Atwood and playwright Tom Stoppard.
Rowling used both sides of her card to hand-write a prequel to her seven-book Harry Potter saga, while Lessing penned a story about the power of reading. Stoppard wrote a short mystery and Atwood was due to fill out her card remotely using a robotic arm controlled by computer linkup.
Organizers refused to comment on the content of Rowling’s Harry Potter prequel, but Hogwarts fans hoping for another book to add to their collection may be disappointed by her signoff.
"From the prequel I am not working on — but that was fun!" Rowling wrote.
Copies of the cards will be put into a book and made available at Waterstones and online in August.
UPDATE: Thanks to Jon for letting us know that the prequel will be about "the fictional wizard's father James and his friend Sirius Black in their younger years."