An EPUB file is the industry global standard for an electronic book, an e-book. The ePub format is rapidly being adopted by online e-book retailer as the preferred file for submission of e-books. It is, essentially, a set of related HTML, XML, image and other resource files in a single-file (zip) format.
The EPUB format is developed and maintained by the International Digital Publishing Forum (IDPF). On May 23, 2011, the IDPF announced the proposed EPUB3 specification is ready for final review with anticipated adoption later this summer. EPUB3 will require XHTML code (which is a stricter standard than HTML) and adds greater formatting capability, video and enhanced global typography (e.g., support for Mandarin Chinese and Arabic alpahbets).
As yet, the ePub file format is not required by reatailers such as Amazon and Barnes & Noble but that may change very soon. Apple does require publishers to submit ePub files.
Online conversion services that promise to convert your Word document to ePub often have unpredictable and unsatisfactory results. These automated programs make a valiant effort to accomplish an extremely complex task. Upzip a sample ePub from their site and you will immediately notice random file, bookmark, and hyperlink names. If you start with a very simply formatted novel, you may get an acceptable result. In the real world, things are never quite that simple.
To learn more about the EPUB3 specification click here.