DRM stands for Digital Rights Management and is a means by which content owners and distributors restrict the use of their digital content. DRM however isn't a new concept and has been used to protect software installations for many years. When you enter your serial number to activate Windows for instance you are using a form of DRM.
The issue of protecting content is now the subject of a heated debate between content creators, publishers and distributors and is coming to a head thanks mainly to the market dominance of Apple's iTunes music stores, by far the largest distributor of DRM'd content.
Presently many different distributors use different DRM systems, most of which are incompatible with others. For example, if you buy a song from iTunes it will only work in iTunes and on the iPod and if you buy a song through a Windows based music store it won't work on the Mac or the iPod, or many other digital music players like Microsoft's newly released Zune.
DRM is confusing for consumers and is being blamed for the slow growth of digital content sales. In an effort to grow the industry Apple, in partnership with EMI have announced they will begin selling DRM free music beginning in May. The new DRM free music will be 30% more expensive however will be encoded at a higher bit rate, providing consumers with higher quality downloads.
Other content publishers however take comfort in DRM as it lets them control who listens to their music. In practice however media piracy is so prevalent that DRM hasn't decreased it, in fact some believe it has had the opposite effect. Additionally, the anti-DRM lobby makes the point that CD's don't have copy protection, so why should digital music.
As a consumer it's fascinating to watch this debate from the side lines. It's also interesting to watch the shift in power from the incumbent content publishers to the new age distributors who deal directly with their customers.