AZW, probably standing for Amazon Word, is an eBook format used exclusively on the Amazon Kindle and compatible Kindle software on PC's or iPhones. It is basically the MOBI format using the high compression option. The AZW extension is used on both eBook files that are protected with DRM and files that are not protected. http://wiki.mobileread.com/wiki/AZW. The .azw3 extension is (presumably) for version 3 of this format.