Filk songs, in SCA usage (the term predates the SCA), are songs that are either old songs (either traditional, medieval, or modern songs) with new words, or just brand new songs, relating to SCA activities and lifestyle or amusing medieval content. They are often sung at Bardic Circles, while getting drunk around campfires, or on very very long car trips to camping events. Some songs are incredibly witty and amusing, others are so awful they make your head bleed, but either way, it's a fun activity for SCAdians to get involved in.
Not all new songs are filk, although to be honest the definition of what constitutes filk is fluid and fuzzy. A song written in a period style and with few or no OOP references is not neccessarily a filk song simply by virtue of having been writing in 2006 rather than 1066. It might just be a song. For a more detailed discussion, try http://en.wikipedia.org/wiki/Filk