Filk can be broadly defined as music written within a Science Fiction of Fantasy theme, although there is really no universally accepted definition. Under this definition, Wizard Wrock is technically a form Filk, but is usually distingushed by the fact that Wizard Wrockers write original music for their songs. Most Harry Potter Filks are written to borrow the music of some popular (or perhaps obscure depending on to whom one is talking) tune.
There are several web pages interspersed throughout the internets which deal with Harry Potter Filks.
Harry Potter Filks is a compendium of filk lyrics, including some complete musicals based on the Harry Potter stories.
Swish and Filk is another important website that catalogs the Filks of several artists. It was started by The Leaky Sisters, who banded together in appreciation of Filk as a from of expression within the Fandom. Lyrics and downloadable mp3 versions of many filks are available at this site. Some time in 2009, Swish and Filk merged with Filkcast: PotterStyle, a podcast devoted to Harry Potter Filks and has become the home for both entities.