The Kindle supports two DRM-laden formats, AZW and TOPAZ. The vast majority of Kindle ebooks are AZW's, and this is essentially identical to DRMed MOBI. All AZW ebooks can be made into DRM-free MOBI using modidedrm and the Kindle's PID (e.g. from KindleFIX, see
Visual Kindle Guide).
The TOPAZ ebooks are only readable on the Kindle.