EPUB (short for electronic publication; alternatively capitalized as ePub, EPub, or epub, with "EPUB" preferred by the vendor) is a free and open e-book standard by the International Digital Publishing Forum (IDPF). Files have the extension. epub. EPUB is designed for reflowable content, meaning that the text display can be optimized for the particular display device. The format is meant to function as a single format that publishers and conversion houses can sell in-house, as well as for distribution and sale. It supersedes the Open eBook standard.
*courtesy of Wikipedia