Just the format if you distribute it in a .ibook file. You are free to distribute the content as a Kindle or PDF or ebook or whatever.

Absolutely no one will be forced to use the new .ibooks format or the tools that create those files. If you wish to publish a non-exclusive EPUB on the iBooks store as well as on Amazon, Nook, etc, you are welcome to do so. Nor do I personally think that Apple will come after anyone who shares material between .ibooks editions and EPUB ones. I am, obviously not a lawyer, but I believe Apple is protecting and charging for use of its format, not aggressively seizing content.

Apple clarified the EULA here:

The change is an important one though, clarifying that Apple has rights over the format a book is in, not the content. If someone wants to charge a fee for an .ibooks formatted file, that document can only be sold or distributed through Apple, and that work will be subject to a separate agreement with Apple.