The International ISBN Agency has decided that it's time to reconsider the standard procedure for assigning ISBNs to ebooks. As a first step, they created a survey in order to collect feedback. You can find the survey
here.
An ISBN is a 10 (or 13) digit number that's basically a serial number for books. If a book came out in multiple formats (paperback, hardback, etc), each format had its own ISBN. This applies to ebooks as well; an ebook has a different ISBN for each format sold (LIT, Epub, Mobi, etc). The ISBN Agency has decided to reconsider this decision.