The 'ePUB' or .epub is a standard for ebooks from the International Digital Publishing Forum. ". ePUB " is the file extension of an XML format for reflow able digital books and publications. It is an arrangement of several other standards (mainly: XHTML, CSS, XML, NCX, DCMI).
". ePUB " is composed of three open standards, the Open Publication Structure (OPS), Open Packaging Format (OPF) and Open Container Format (OCF), produced by the IDPF. There are three parts addressing: content, package metadata, and archive (OPS, OPF, and OCF). It is powerful, straightforward, and non-proprietary. It consists of basic XHTML for the book content, XML for descriptions, and a re-named zip file to hold it all in.
Adobe Digital Editions (ADE) is one of the first readers for Epub publications.