EPUB is an electronic book (or "ebook") saved in a standard open format. It can be read on a PC using ebook software or on an e-reader, such as the Barnes and Noble Nook, or Sony Reader.
EPUB files are saved in a compressed format that includes multiple XML files. These files define both the structure of the book, as well as the content. The current EPUB standard is developed and maintained by the International Digital Publishing Forum (IDPF).
Important: While EPUB files are saved in an open format, many downloaded books include DRM protection, which limits the devices on which the book can be viewed. Therefore, if you cannot open an EPUB file on computer or e-reader, the device may not be authorized to view the ebook.