The 'K' stands for Köchel, which is the name of the catalog of Mozart's works and, I assume, the person who cataloged them.
http://www.mozartproject.org/
You will also see BWV numbers or Schneider listings for the works of J.S Bach.
http://www.mozartproject.org/
You will also see BWV numbers or Schneider listings for the works of J.S Bach.