azw3 The American company Amazon is a recognized powerhouse when it comes to all things related to books. Their proprietary eBook readers dubbed the Kindle which was first released in 2007 is a testament to this. The .azw file format is the filename extension associated with eBooks designed to be read on Amazon's proprietary Kindle device and through their free but closed source Kindle software for smartphones, PCs, and Macs. The format is incompatible with any device other than kindle and its associated software. In order to open files saved in this format on non-kindle devices, one would have to convert the file to another format such as the open ePub format or PDF. Some converters can also convert other formats into .azw files which can later be opened on a kindle device or application.
fb2 The .fb2 filename extension refers to the open XML based eBook format known as FictionBook. The file format was developed by Dmitry Gribov with an initial release in 2004, quickly gaining popularity in Russia. The format is open which in this context means it has no compatibility with digital rights management technologies. The structure of the format splits the contents of the data into three sections; title information, XML document information, and finally information about the paper edition which is optional.