Digital Rights Management is a term used to refer to any method of protecting any content that is produced in a digital form, although most people seem to focus on the music aspect, however solutions exist for HTML, XML, ASCII Text, Quark, PDF and Microsoft formatted files (Word, Excel, etc.).
DRM solutions tend to work in a rights label basis, with the rights expressed as metadata, that are associated with content being distributed.
The metadata is information about the file itself, for example, who the created the digital work. A rights label contains the rights (print, view, copy, etc.) associated with the content and the conditions (fee, time, access) that are enforced for those rights. It also contains summary information regarding the content that can be tied to other systems (DOI number, etc.).
The rights label is read by a rights server and used to create the permit issued for the content.
In an ideal world there would be a platform agnostic solution for this, however this holy grail as yet to arrive, and so DRM solutions are usually platform specific.
A proposed standard for the expression of rights, XrML (eXtensible rights Markup Language) has been proposed, as a joint effort between Xerox Parc and Microsoft.