Long ago I found this web page about the ballad:

http://www.english.upenn.edu/~afilreis/88/ballad.html

Follow the link to "Ballad Stanza" as well. You'll find that many favorite "folk" songs do fit this definition and it explains why some songs "sound like" other songs - because they are written in this pattern or something very close to it. Combine that with "telling a story" and you will come up with many favorite "folk" tunes.