A Work refers to a musical composition, that is, the instructions to performers about what sounds and silences to perform on what instruments.
It does not refer to the act of performing or the actual sounds of the music — that is a musical performance. Nor does it refer to the recorded signal fixing the performance in a persistent medium - that is a track.
The concept of work is important in classical music, because in many cases there are multiple recordings of the same work. The classical style guide specifies that release titles and track titles identify the work.
As of January 2010, the MusicBrainz Database does not have a specific work entity in the database schema, however, the next generation schema that is being implemented with the next server release will have work support.