FictionBook is an open XML-based e-book format, which originated and gained popularity in Russia. It is supported by e-book readers such as FBReader, AlReader, Haali Reader and okular. FictionBook files have the .fb2 filename extension.
FictionBook is based on the XML standard from W3C, and FictionBook documents are well-formed XML documents, conforming to the FictionBook schema. The FictionBook format does not specify the representation of a document, describing its structure instead. For example, there are special tags for epigraphs, verses and quotations. All the ebook metadata, such as the author name, title, and publish information, is also present in the ebook file. Hence, the format is convenient for automatic processing, indexing, and ebook collections management. Moreover, it allows automatic conversion into other formats.
Binary data such as images are stored inside the special XML tag (binary) at the end of the file and converted to Base64.
Complete description of this format and its XML schema specification can be found on Fictionbook portal.