I do like the explanations of Uncle Dave, Jim Carroll and Sandy McLean. I would add that some time in the late 30s/40's the music industry misappropriated the term to refer to any slow love song they wished to perpetrate on the public.

But further, we should differentiate between the longer narrative, Child/Big bBallads,Muckle Sangs and the more down to earth folk ballads of later creation. I'll give examples if anyone's interested in pursuing this - Tom Hall