DRM: When you absolutely, positively need to get screwed because your home media system is too good.
DRM: Digital Rights Management. A technology which uses strong crypto to control whether or not a particular computer is permitted to decrypt and play back a particular media file. The idea is that unless a given box has been outfitted with a particular certificate, it doesn't matter if the files are shared or not, only the system for which the certificates were issued could play them back, assuming that the company that provided the certificates didn't decide to revoke them or something.
The 'or something' is the operative part of what screwed one Davis Freeberg not too long ago: An afficionado of HD (High Definition) media, he bought a brand-new Samsung SyncMasterTM 226BW HD display for his computer, the better to watch movies bought, paid for, and downloaded from Amazon and Netflix. As it turned out, the combination of an HD monitor and graphics card in his computer was a little too high-def for comfort and the DRM management software in his computer provided by Netflix refused to let him watch anything unless it erased all of the licenses already stored, forever preventing him from watching anything that he'd already downloaded. This is possibly due to paranoia on the part of Hollywood - I guess if your display is too good, someone could point a video camera at the screen, make a recording, and then distribute that. One would suppose that the quality would be marginally better than what you'd buy at a bootleg movie sale in downtown New York City.
Whether or not those deleted files could be recovered and put back is something that I don't know. Freeberg mentions that there is a way to back up the DRM certificates and restore them, but it's a long and tricky process.
Netflix's recommendation? Downgrade to VGA.
My recommendation? Don't patronize any companies that force you to take a hot DRM enema just to watch a movie. Take your time and money somewhere else.