Correct me if I'm wrong but I always understood a ballad was a poem or a song which told a story but always came back to a repeated refrain. You can almost always identify the song by the refrain. Some times the refrain has variations but it is always recognizable. Some examples are (and forgive me if they are not all "folk"):

"...But the point of a gun was the only law
    that Liberty understood and when it came
    to shootin' straight and fast, a law book was no good."

"...But way up there He'll hear our prayer and
    show us where there's water, cool, clear, water."

"...Oh Puff the magic dragon lived by the sea
    and frolicked in the Autumn mist
    in a land called Honalee (?)"

"...No the circle will be unbroken, by and by Lord
    by and by..."

"...Hear a few grey Federales say, 'Could have had him any day,
    We only let him slip away," out of kindness I suppose."

"...Hey Mr. Tambourine Man, play a song for me. I'm not sleepy   
    and there is no place I'm going to..."

I think it is sometimes hard to distinguish between a refrain and a chorus. Chorus is a separate topic and perhaps should have it's own thread. Let's see how the discussion goes.