Take a look at line 6:
And smiled among the winter's snow.
That, ladies and gentlemen, is what we call a perfect line of iambic tetrameter. That means it consists of four iambs ("and smiled" is one, "among" is one, and so forth), all in a neat little row. daDUM daDUM daDUM daDUM. Cute, right?
Unfortunately, Blake was never one to keep things neat and pretty. For an example, we'll turn to line 1:
A little black thingamong the snow.
You'll notice that there are an odd number of syllables here, which means our line isn't perfect, per se. The first two groups are iambs (daDUM daDUM), but the next two contain a stressed syllable followed by an unstressed syllable—DAdum. This, folks, is called a trochee. Then, the line ends with a single stressed syllable—snow. It's like a little bonus. In a line that's supposed to have eight syllables, we get nine. Sweet.
Let's look at one more line:
And taught me to sing the notes of woe.
The first, third, and fourth groups are iambs; but what about the second group, which contains two unstressed syllables followed by a stressed syllable—dadaDUM? This is called an anapest, and Blake is a big fan of droppin' those suckers in his iambic lines. Look for more of them in this, and other Blake poems.
Like many of Blake's short lyrics, the poem rhymes, and those rhymes come in a pattern—a rhyme scheme. Since this poem also consists of three four-line stanzas (a.k.a. quatrains), we can break the rhyme scheme down by stanza. It looks a little something like this:
Stanza 1: AABB
Stanza 2: CACA
Stanza 3: DEDE
All the lines marked "A" rhyme with each other, all the lines marked "B" rhyme with each other, and so on. You'll notice that Blake sets up a pattern of rhyming couplets in the first stanza, but then promptly abandons that rhyme scheme altogether for the next two. What's up with that?