DRM, or Digital Rights Management, is the system whereby restrictions are placed upon the use of your content by the people who made or sold you said content.
It affects all mediums, from e-books to movies, but it's especially important/hated in the video game industry for the way it's implemented on certain games, especially those on the PC.
For those angry enough about DRM to want to do something about it, well, today is International Day Against DRM. You can check out the movement's site below for details on local events or ways you can campaign against it online.
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International Day Against DRM - May 4, 2012 [Defective by Design, via Boing Boing]