DRM stands for Digital Rights Management, and is, simply put, any technology that is used to restrict and manage content.  DRMs on DVDs make it so that they cannot be simply put in to your computer and copied, and DRMs on a program like Windows XP itself make it so that you can't just put Windows XP on every computer you come across.  Of course, these things still happen as DRMs are ineffective.

Richard Stallman Explains DRM: Here

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