DRM stands for Digital Rights Management.
DRM doesn't mean just basic copy-protection of digital content (like ebooks, MP3s or DivX videos), but it basically means full protection for digital content, ranging from delivery to end user's ways to use the content. somehow DRM system needs to know when the copying is allowed and when not -- users also have rights to make copies to their closest relatives, etc. So, normally this has been solved by allowing "hops" -- original file can be copied, but the copy of the original file cannot be copied any further. Obviously this also causes problems, if user accidentally deletes the original file, but still has the legal copy of the file.
Digital Rights Management (DRM)1 is an umbrella term referring to any of several technical methods used to control or restrict the use of digital media content on electronic devices with such technologies installed. The media most often restricted by DRM techniques include music, visual artwork, and movies. Some digital media content publishers claim DRM technologies are necessary to prevent revenue loss due to illegal duplication of their copyrighted works. Civil libertarians, in contrast, argue that transferring control of the use of media from consumers to a consolidated media industry will lead to loss of existing end-user rights, as well as stifling innovation in software and cultural productions. No current DRM technology includes a mechanism to enable fair use rights per se, though some DRM methods allow acts of copying which may coincidentally align with legal use rights. Arguably, a technology cannot, in principle, know what legal restrictions and rights apply in a specific jurisdiction, usage context, under an external contract, or to an individual author, owner, or publisher.