The .FB2 file extension refers to Fictionbook files, which is an xml based file format used for E-book format structuring. The file format is based on Open Xml and hence is very popular on the internet.

The file structure of Fictionbook files is unique in the sense that it does not describe the contents of the file but instead just focuses on the structure of it. The format supports storing the ebook metadata and hence all the extra information about a book can be embedded with the file itself. Often, a .FB2 file is packaged inside a zip archive.The format is quickly gaining popularity has plenty of hardware and software support implementations.