Digital Rights Management
From Mac Guides
Digital Rights Management is the encryption or alteration of a file to keep it from being copied or distributed in ways that the author or license-holder did not intend. The most typical use of this is in legal music download services such as FairPlay in the iTunes Music Store or services like Napster. Currently the file types that most commonly use DRM are aac, mov and wma (which is Windows only).
More detailed information can be found on Wikipedia's DRM Page.