What is an FB2 file?
FB2, or FictionBook 2, is an open e-book format built on XML.
It came out of the Russian-speaking e-book world in the early 2000s and is still widely used there. Outside it, FB2 is the format people meet when they download a book from a Russian library site and find their reader will not open it.
What is inside an FB2 file
An FB2 book is a single XML file. That is the whole design, and it is what makes the format unusual.
Where an ePub file is a ZIP archive holding many documents, an FB2 book puts the text, the structure and the metadata in one file, with images stored inside it as base64 encoded data. There is no package document to maintain and no container.
The format describes a book semantically rather than visually. A tag says this is a chapter title, this is an epigraph, this is a poem, and the reading app decides how those should look. FB2 has no styling of its own in the way ePub has CSS.
You also meet .fb2.zip, which is exactly what it sounds like: the XML file compressed. Most readers that handle FB2 open the zipped form too.
What opens an FB2 file
FB2 support is common in dedicated e-reading software and rare everywhere else.
Calibre opens FB2 on Windows, macOS and Linux, and converts it to other formats. FBReader, as the name suggests, was built around the format and runs on most platforms. PocketBook e-readers support FB2 natively, and so do several other European readers.
Kindle does not. Neither does Apple Books. If your file is FB2 and your device is one of those, conversion is the route rather than a setting.
FB2 compared with ePub
Both are open formats, both reflow text to the screen, and both are XML underneath. The differences show up in what they are for.
FB2 is simpler and stricter. A book is one file, the structure is described rather than styled, and that makes it excellent for plain prose and for building libraries that can be searched and reformatted freely.
ePub is a packaging format for web content, so it carries CSS, embedded fonts, audio, video and scripting. Anything where the presentation matters is far easier in ePub, and that is why it became the international standard while FB2 stayed regional.
For a novel, the reader will struggle to tell the difference. For anything illustrated or designed, ePub wins by a distance.
Converting FB2
Calibre is the usual answer here too. It reads FB2 and writes ePub, which is the conversion most people want, and the result is normally clean because both formats describe structure rather than fixed pages.
Calibre: calibre-ebook.com
The one thing worth checking after a conversion is the images, since FB2 stores them inside the XML and a lazy converter can drop them.