Appart from the obvious "common person's music" definition that is forthcoming, folk music crosses cultures to the point where virtually the same song has been delevoped around the globe, independently.

It stems originally from "acoustic" musical instruments (which ones aren't "acoustic") and anything that one might make sound with that didn't require much training. Folk music has a very simple rhythm, melody, and harmony, and is/was generally considered from a traditional base (ie. learned from rote from ones ancestors).

The above desciption (both unfortunately and fortunately for different reasons) describe most of todays popular music. ('cept for maybe the independent part)