DRM, which means Digital Rights Management (or as many say, Digital Restriction Manager), restricts how you can use media and even devices that you have purchased. DRM is used by many companies including Apple, Microsoft, and Sony.
When a file is protected by DRM the number of computers it can be used on can be limited and many other things. I would guess this would include things like the amount of time and the time of day it can be used.
Along with restriction of use, if a DRM server is shut down, the files are no longer usable. For example the Yahoo! music service was shut down recently and now people who purchased music from Yahoo! can no longer listen to the music they downloaded. Apple offers both DRM protected and DRM free files.
Apple’s DRM Restrictions:
(Source: http://en.wikipedia.org/wiki/FairPlay )
By summer time, Apple will no longer use DRM with iTunes!
There are many biased websites about DRM like “Defective By Design” (http://defectivebydesign.org/).
Personally, I think that DRM shouldn”t be used, and that perhaps people shouldn’t try to protect things like that. I really think that with the current copyright laws and the ignorant people that the market relies are good enough because the owners of these intellectual properties would make enough money without having to tell who can use it and keeping certain people from using it in this way.