DRM stands for Digital Rights Management and it is now (spring 2002) the hot topic among content owners and technology companies alike.
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.
If we speak about music DRM, companies wish to develop a product which would allow record labels to sell copy-protected audio tracks over the Internet, so that only the buyer could be allowed to listen the tracks. This is technically difficult, because traditionally, national laws require content owners like record labels, to grant "fair use" rights for products consumers buy. This means that user has to be allowed to make personal copies of the purchased music, in order to use the music in car, in portable digital audio player, in his/her laptop computer, etc.
So, 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 :-)
In April 2007, EMI became the first of the major music labels to stop selling their music with DRM on the iTunes music store. This is a massive step forward in the direction of protecting consumer's rights. We will see how their step will influence other music labels in the future.