The EPUB format is a standard ebook format recommended by The International Digital Publishing Forum. An EPUB ebook has the extension of ".epub", but it is essentially a zipped file. You can rename ".epub" to ".zip" and unzip the file.

The unzipped EPUB files may be divided into three categories: XHTML documents, packaging files and container files. XHTML documents are similar to, but stricter than, HTML documents. They can have ".htm", ".html" or ".xhtml" extension. In an EPUB ebook, the main contetns are placed in these XHTML documents while the packaging files (.opf and .ncx) hold information about the book, such as author, publisher and the organization of XHTML files (parts, chapters and sections). The container files include other information such as the location of the pakaging file (.opf) and the encryption method (if any) of XHTML files.

Currently, the epub format can be read by Adobe Digital Editions, FBReader and Mobipocket.