The ballad is a form of poetic verse that tells a story (a narrative). Ballads were stories set to music and meant to be danced to. Typically, a ballad has four-line stanzas (quatrains) and has a rhyme scheme in which the second and fourth lines in each stanza are rhymed but not the the first and third (A-B-C-B). The meter of the lines tends to be alternating iambic tetrameter (eight syllables) and iambic trimeter (six syllables), though there are variations. Note that "iambic" refers to the pattern of an unstressed syllable followed by a stressed syllable.

"The Demon Lover" is an old Scottish ballad in the traditional sense. It certainly is a narrative. It tells the story of a young wife and mother whom the Devil tries to tempt into running away with him by offering riches.

In terms of poetic form, you can easily see that the traditional rhyme and syllable pattern are those described above (rhyming second and fourth lines of each four-line stanza with alternating eight- and six-syllable lines in iambic pentameter).