"In technical prosodic parlance we could say that most ballads present in quatrains of alternate cross-rhymed iambic tetrameter and trimeter. However, since the ballad is a swinging, popular form derived from song and folk traditions, it is much better described as a form that comes in four-line verses, usually alternating between four and three beats to a line. The word comes from ballare, the Italian for 'to dance' (same root as ballet, ballerina and ball)." - Stephen Fry, The Ode Less Travelled.

Fry's example is:

"There's nothing like a ballad song,
For lightening the load,
I'll chant the buggers all day long,
Until my tits explode."