Digital rights management (DRM) is a generic term that refers to access control technologies that can be used by hardware manufacturers, publishers, copyright holders and individuals to try to impose limitations on the usage of digital content and devices. The term is used to describe any technology which inhibits uses (legitimate or otherwise) of digital content that were not desired or foreseen by the content provider. The term generally doesn't refer to other forms of copy protection which can be circumvented without modifying the file or device, such as serial numbers or keyfiles. It can also refer to restrictions associated with specific instances of digital works or devices. Digital rights management is being used by companies such as Sony, Apple Inc., Microsoft and the BBC. 
What is wrong with DRM Edit
The problem as it is mentioned in further paragraphs at the Wikipedia article, is that you buy a product, but you do not have total control over it. It's like "leasing", but the websites and companies say that you are "buying".
Another way to define DRM is when you restrict a person that purchased something, for example a song on iTunes, to use only specific product (hardware or software) with a specific file type or codec, without allowing you to use the purchased product (song) in any other form of hardware or software.
The pirated response Edit
Pirated media that circles around the Web, although unlawful, they bring certain freedoms. For example, DRM is not used, so people can actually use their hardware or software of preference to play and enjoy certain media.
- ↑ Source: Wikipedia http://en.wikipedia.org/wiki/Digital_rights_management