http://freedesktop.org/wiki/Standards_2fshared_2dfilemetadata_2dspec

There are a number of metadata frameworks and indexers such as Beagle and Kat as well as a new freedesktop system Tracker, which is based on this spec and is currently under development. These frameworks provide a rich source of metadata about files including such things as the author of a document or the artist of an mp3 file. The purpose of this specification is to define a common metadata naming scheme that each framework can implement to allow applications to tap into this wealth of information. Some examples of interested applications would include filemanagers that want to display and allow editing of this metadata as well as providing integrated search functionality and virtual folder capability (IE folders whose file contents are defined by metadata rather than physical location). This specification will define a common set of “well-known” metadata.