EPUB: Electronic Publication
What is an ePub?
EPUB (electronic publication), sometimes ePub, EPub or epub, are files with the extension
.epub. It is also "a free and open e-book standard, by the International Digital Publishing Forum (IDPF)." EPUB supports reflowable, or responsive content, which means that optimisation for particular display devices is built into the design. ePub is meant to function as a single format for use by both publishers and conversion houses, and also for distribution and sale.
Technically, an ePub is just a compressed ZIP file, containing a bunch of XHTML and XML, so it's easy for anyone to make your own with a wide range of tools (including the most basic text editors) and this simplicity built on top of open standards is an attractive solution for the publishing industry to support it.