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).
Kindles can read a variety of non-proprietary, non-DRM'ed formats. They read TXT, HTML, JPEG, and non-DRM Mobi files directly.
We've known for some time already that Amazon's AZW files are actually Mobi files, but Amazon didn't share Kindle's Mobi PID which would allow one to buy encrypted Mobi books for 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.
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