FictionBook is file format created for e-books. His extension is .fb2 or .fb2.zip. This file format is open, free editable based on XML. His main advantages are awesome support of text formatting (bold or italic text, underline, titles, etc.) It also supports file inserting like tables, pictures, citations and the least but not last text marking.
It´s release was done in 2004 by Dmitry Gribov, who is Russian programmer. Since that it was updated many times. Its latest version was released in 2008 and it was marked as 2.21. From .fb2 format was derived .fb3 There´s plenty of applications, which can open these files. The best known and the best one is FBReader, which even in its name shows, that it is especially created for this file format. Then there are programs CoolReader, Okular, etc. This file type is also readable in e-books.
As we wrote above the best program to open this file format is FBReader.