Ballads are everywhere in popular music. Think slow, moving, and emotionally powerful numbers, like this one. (There's even a subgenre of power ballads, like this one). In poetry, a ballad is simply a poem that sounds like a song and can be easily set to music. If you find yourself bobbing your head or tapping your toe to poems about love, you know you're more than likely reading a ballad.
What makes a ballad like "Down by the Salley Gardens" sound so catchy is the speaker's use of simple alternating rhymes and the use of iambic trimeter. An iamb is just a two-syllable pair where the first syllable is unstressed and the second is stressed (say "allow" and you'll hear one in action). Iambs are super-common in ballads because, when strung together, they create a rhythm that makes a song easy to remember. Take a look at line 2: "my love and I did meet." Hear that daDUM daDUM daDUM pattern? That's three iambs in a row, or iambic trimeter.
Still, this patter is only mainly how the poem's meter is organized. Usually, it's the even-numbered lines that have the iambic trimeter, but even so you can't exactly set your watch by it (check out lines 6 and 14 for just two exceptions). So, why not keep things locked down all the way through? Well, this poem is about a romance gone bad. Our speaker at first seems to hint at a love connection, but he just won't listen to the girl, and so he's left out in the cold by the poem's end, crying huge sad tears and wiping his nose on his sleeve. In a poem that's about not getting together, we think it makes perfect sense that this neat, predictable rhythm gets pummeled and ignored from time to time—just like our poor speaker.
The same can be said for this poem's rhyme scheme. We get glimpses of harmony with the regular end rhymes in the even-numbered lines, but we don't get the same thing in the odd lines. Instead, we get no rhyme or, at times, slant rhyme, like "river" and "shoulder" (lines 9 and 11). In this case, the words only kind of rhyme, approaching harmony but never quite getting there. Gee, does that remind you of any Irish couples hanging out in some pretty gardens at all? Hey, us too. This poem—in both meter and form—gives us a glimpse of perfect unity, but ultimately shows us that it's just not in the cards.