Hack let you convert encrypted Mobipocket files that were intended for another reader so that that work on Kindle.
DRM refers to the fact that you can't view Amazon Kindle books on other devices (including your computer), or on other Kindles - i.e. no sharing, the purpose of DRM.
The Kindle is already able to read more than just Amazon DRMed content, it can handle a number of formats.
Typically ebook readers are larger than PMP's with larger screens. If you're going to read for a few hours, then the screen of a typical PMP becomes pretty eye-straining (due to size, contrast, and brightness).
A revolutionary portable reader that wirelessly downloads books, newspapers, magazines and blogs to a crisp, high-resolution electronic paper display that looks and reads like real paper, even in bright sunlight.