The ballad is a poetic form based in song or folk tale. Usually written in quatrains (that's stanzas of four lines), and usually with an ABAB rhyme scheme (per stanza). So if there are two stanzas, it might be ABAB CDCD, for instance. That said, it can also have the rhyme scheme ABCB, and can also be in six-line stanzas. For instance, Lewis Carroll's The Walrus and the Carpenter is in six-line stanzas using the rhyme scheme ABCBDB.
With the ballad, as with the limerick, the important thing in a ballad is not the precise number of syllables, but the number of stressed beats. A syllable is, well, a syllable. A beat is a stressed syllable. Not entirely sure what I mean? Have a look at the first two lines of, say, Amazing Grace:
Amazing Grace, how sweet the sound
that saved a wretch like me.
The beats in the line are the long-sustained words in the song. Thus:
aMAzing GRACE, how SWEET the SOUND
theat SAVED a WRETCH like ME
And yes, ballads almost always alternate four-beat and three-beat lines in that manner.
What's that? You say you couldn't read those words without hearing the tune in your head? Well, then, you're in luck: A lot of ballads can actually be sung, even if they were never truly meant to be. The best-known and most popular song choice for this is that of Amazing Grace, although The Yellow Rose of Texas works, and so does the theme song from Gilligan's Island. Don't believe me? Go ahead and sing the lyrics to Amazing Grace to the tune of Gilligan's Island or, for a more absurd result, sing the lyrics to Gilligan's Island to the tune of Amazing Grace. (When I do that second bit, I'm immediately reminded of my favorite Star Trek movie ever, Galaxy Quest, and in particular the scene where the Thermians moan with despair over the thought of "those poor people." But I digress.)
Many of Emily Dickinson's poems were ballads. Here's one for you (number 21, to be precise) that demonstrates her facility with the ballad form:
He ate and drank the precious words,
His spirit grew robust;
He knew no more that he was poor,
Nor that his frame was dust.
He danced along the dingy days,
And this bequest of wings
Was but a book. What liberty
A loosened spirit brings!
So -- which tune did you settle on? For me, it's usually Amazing Grace, but the others are so much more fun.
I'm off to write my own ballad today. And you?