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.
mobi The .mobi filename extension is an eBook file format developed by French company MobiPocket for their ebook readers for mobile phones and PDAs. The company was later acquired by book reseller and online store company Amazon in 2005, a deal which included the mobi format asset. The mobi file format is mostly associated with the mobipocket software package as well as early versions of Amazon's kindle ebook reader device. The format is considered cross platform given its compatibility with several devices including most smartphones that have compatible readers installed. But although mobi files still exist in circulation, the format was superseded by Amazon's AZW format when support and further development of mobi ceased in 2011.