This file type developed by company Amazon is format especially created for eBooks. Basically it is PDF file with certain adjustments. Essentially it is whole new format that means it is the only one officially supported format for Kindle on PC and Mac. Sometimes it is called Print Replica, which means that whole page is loaded ahead and then it hasn´t to load anything. This approves the fact that it is derived from pdf format. Main advantage of this format are certain formatting tools like highlighters or sync options.
Release of this format was few days after the release of application Kindle for PC in 2011, but first mentions about the new format were released on 24th August 2011.
There are also several programs, which can read these files. To the best known belong Amazon Kindle, he´s available for Windows, Mac and even for Android, iOS and Windows Phone. Then there is Calibri, which is best known on Linux, but you can use him also on Windows and Mac OS. To the best known e-book readers belongs Amazon Kindle.