The EPUB (electronic publication) format is an open standard from the International Digital Publishing Forum (IDPF), an association that has been formed from members of the digital publishing industry to develop a reliable standard for this exciting new form of publishing.
EPUB shouldn't be confused with the Digital Magazine format that's emerging; EPUB is designed for re-flowable content that will be optimised for the gadget displaying it, such as the the iPad. If you're used to working with HTML standards or PDF accessibility, then you'll be quite familiar with the concept of re-flowable content, but don't worry if you're not because it's a fairly easy concept to grasp.