An AZW3 file is an eBook created in the Kindle Format 8 (KF8), Amazon's successor to the .AZW format. It supports newer features such as HTML5 and CSS3 support as well as many other formatting capabilities. AZW3 files often include DRM protection and are used as the download format for many Amazon eBooks.
KF8 eBooks can be created with KindleGen, a free command line tool provided by Amazon. AZW3 files can be previewed with Kindle Previewer, a Kindle software simulator.
NOTE: KF8 eBooks also use the .MOBI extension. The format downloaded from the Amazon store, either AZW3 or MOBI, depends on the device being used to download and read the eBook.