EPUB (short for electronic publication) is a free and open e-book standard by the International Digital Publishing Forum (IDPF). Files have the extension .epub. EPUB is designed for reflowable content, meaning that an EPUB reader can optimize text for a particular display device. EPUB also supports fixed-layout content. The format is intended as a single format that publishers and conversion houses can use in-house, as well as for distribution and sale. It supersedes the Open eBook standard. An EPUB file uses XHTML 1.1 (or DTBook) to construct the content of a book as of version 2.0.1. This is different from previous versions (OEBPS 1.2 and earlier), which used a subset of XHTML. There are, however, a few restrictions on certain elements. The mimetype for XHTML documents in EPUB is application/xhtml+xml.
FictionBook is a popular XML-based e-book format, supported by free readers such as FBReader, Okular, CoolReader, Bebook and STDU Viewer. The FictionBook format does not specify the appearance of a document; instead, it describes its structure and semantics. All the ebook metadata, such as the author name, title, and publisher, is also present in the ebook file. Hence the format is convenient for automatic processing, indexing, and ebook collection management. This also is convenient to store books in it for later automatic conversion into other formats.