EPUB, short for “electronic publication,” is an open standard format for eBooks.
As an open standard, it has great compatibility across a wide range of eReaders and has become the de facto universal standard within the eReader sector. In contrast, Amazon’s AZW format for its Kindle is a proprietary or “closed” format, which means AZW eBooks can not be read by other eReaders. The Kindle is also the only major eReading device that does not support EPUB (Sony and Barnes & Noble’s latest crop of eReaders all support EPUB, as does Google Books).
Like MP3’s for music, EPUB files can also have “digital rights management” included by publishers who want to restrict the copying of such files.