A lot of "To Helen" is written in a meter called iambic tetrameter. This means that many of the lines contain four (tetra-) iambs. An iamb is a type of beat that contains an unstressed syllable followed by a stressed syllable. (Say "allow" out loud to hear an example: da DUM.) You're probably familiar with iambic pentameter, the most common English meter. Well, iambic tetrameter is very similar, except there are four iambs instead of five. Line 8 is a perfect example:
Thy Naiad airs have brought me home.
If you look at some of the poem's other lines, however, you'll see that they don't always fit into the neat little tetrameter box. Take a look at line 1:
Helen thy beauty is to me.
You will notice that the line ends with two iambs, but what about the first part of the line? The first word contains a stressed syllable followed by an unstressed syllable—i.e., the reverse of an iamb. This is called a trochee, and it sounds like DA dum. The second beat (or "foot," as it is sometimes called) contains two stressed syllables (DA DA). This is called a spondee. So, while we can say that iambic tetrameter is mostly what's happening in the rhythm here, it's certainly not the case throughout.
The same thing goes for the rhyme. "To Helen" contains three stanzas of five lines each, and they all rhyme. However, each stanza follows a slightly different rhyme scheme, or pattern. The scheme for the whole poem is as follows: ABABB CDCdC EFFEF. Each one of these letters stands for a line; all those marked "A" rhyme with each other, those marked "B" with each other, and so on. You'll notice also that there is a lower case "d" in there as well. This is because the word "Greece" only partly rhymes with the word "face"; this is often called a half rhyme, or slant rhyme, and, less commonly, imperfect rhyme or near rhyme. Just like the meter, then, the rhyme in this poem is something that changes as the poem progresses.
Okay, so why in the world does Poe change it up so much? Well, for starters, doing the same thing over and over again gets really, super-duper-boring. But that can't be the only reason, can it? Did you notice that each stanza describes Helen from a slightly different perspective? The first is all about how her beauty makes the speaker feel, the second describes her physical features, and third talks about how she's some kind of heavenly statue. Different perspectives should have different rhymes, don't you think? That makes sense to us!
The other thing to note is that the second stanza is the only one that has a slant rhyme ("face" and "Greece"). Now, Poe was a clever guy, and we'd be willing to bet that he could have found a way to make this work, but decided to leave it imperfect. Why? Well, it shows some doubt on behalf of the speaker. He's not sure Helen is totally, 100% like all the mythological beings to which he compares her. In a subtle way, he's letting us know that this praise is not something that the speaker, himself, buys into. Do you think that makes this poem more believable?