EPUB as a preferred industry open standard format for e-book creation and distribution and is the most common file format for commercially-available e-books. EPUB is an XML based file format designed to reflow text according to screen size whether you are viewing on a large monitor or on a small mobile phones. This format was developed and is maintained by the International Digital Publishing Forum (IDPF, www.idpf.org), a non-profit standards organization. It is Published as .epub and its current version is EPUB 3.2.
Epub is in fact comprised of 3 separate and completely open IDPF standards
• Open Publication Structure (OPS) 2.0, which describes the content markup (either XHTML or Daisy DTBook)
• Open Packaging Format (OPF) 2.0, which describes the structure of an .epub in XML
• OEBPS Container Format (OCF) 1.0, which bundles files together (as a renamed ZIP file)
EPUB format can be read on most reading devices and platforms, including Nook, Kobo, Kindle Fire and Google Books on Android and iPad.
EPUB 3 is a truly international e-book standard. It fulfils the requirements of publishers around the world, including by supporting global language requirements, such as vertical layout, alternative writing, and page progression directions, as well as other typographic capabilities needed to express the wide range of world languages.