Go take a look at Boing Boing: Why Microsoft should get out of DRM. There's an extract from and a link to the text of a speech being given to Microsoft by Cory Doctrow about DRM.
DRM is Digital Rights Management, the software that stops you from having more than 3 copies of anything you download from iTunes and tries to prevent you from copying your CDs. It will force you to base your music purchasing on which type of DRM your PC / stereo / MP3 player supports. It's supposed to protect artistes. It will slowly strangle them.
Lets just hope Microsoft listens to Cory.