eBook created in the Kindle Format 8 (KF8), Amazon's successor to the .AZW format; supports newer features such as HTML5 and CSS3 support as well as many other formatting capabilities; often includes DRM protection; used as the download format for newer 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.