Stands for Bach-Werke Verzeichnis ("Bach Works Catalogue"). A catalogue published by Wolfgang Schmieder in 1950 of the surviving works of Bach, whose indexing system has become a standard for numbering Bach's music. For example, Bach's famous "Coffee Cantata" would be cited as:
J.S. Bach, Cantata, Schwegt stille, plaudert nicht, BWV 211
This is similar to the "K" before numbered works of Mozart, which is the Köchel number (after Ludwig von Köchel) except that they are not chronological. Instead they are ordered by category: cantatas, vocal works, chorales, organ music, Klavierwerke, music for other instruments, and finally the Musical Offering and The Art of the Fugue.
In very early citations you will sometimes see "S" or "SV" for Schmieder number or Schmieder Verzeichnis
For a complete list
by BWV number, see www.jsbach.org/bwvlist.html.