It was published in three volumes (in German) by Bernd Baselt between 1978 and 1986, and lists every piece of music known to have been written by George Frideric Handel.
The catalogue also includes the first few bars of each piece and large amounts of factual information including manuscript sources, early prints, photographs, spurious works, etc.
The HWV thematic catalogue serves as the modern numbering system for Handel's compositions.
There are gaps and anomalies in the HWV numbering system, so the above table is only useful as a guide (see the List of compositions by George Frideric Handel for more details).
The HWV numbers do not imply that Handel wrote exactly 612 works.