Translation: A ballad is a story sung with music. Ballads often include choruses, which can be sung between verses or even attatched to the beginning or end of each verse. Ballads tell a story in a rather peculiar way: rather than describing events one after the other, like a normal story, a ballad will jump from place to place in a story, often telling it from a first person perspective, and leave the listener to catch up. The story is also quite formalized--common motifs, situations, phrases, etc. pop up over and over again.
The ballads described in this page are popularly known as "border ballads". They were sung, for the most part, along the rural areas of the Scottish border, and are a different animal entirely than the broadside ballads so popular in the late 1500s and 1600s. (Broadside ballads were songs with new lyrics put to popular tunes, transmitted by writing rather than orally, and primarily an urban phenomenon.)
The border ballads were originally an oral style, and their form reflects this. Various forms of repetition and formalized phrases were used to help the singer remember the ballad, and the wide variety of versions found for one song stem from the fact that ballads were passed on by ear. The ballad that was eventually written down in a book wasn't exactly the one being sung; Each singer changed the ballad slightly, adding or changing a word here or there, and making slight, perhaps unconscious alterations to the tune. Ballad singing was a living tradition that evolved with every performance.