First of all, "London, 1802" is a sonnet. This means a few things off the bat: it's written in iambic pentameter (or, in this case, a pentameter that's most often iambic). What, you may ask, does this mean? We can break these words apart and see. Pentameter means that there are five metrical "feet," or building blocks, in every line. In this case, the word iambic tells us that each unit is a two-syllable group consisting of unstressed syllable followed by a stressed one – da dum. Try saying it out loud – da dum da dum da dum. Good! Don't worry, nobody saw you.
Let's get back to business. Since every foot is two syllables, we can do some simple math and figure out that all together there are ten syllables in each line of the poem. One of the few flawless lines of iambic pentameter in the poem is line 9 – let's say this out loud, too:
Thy soul was like a Star, and dwelt apart.
Awesome. We foresee a great future for you as a popular reader of books on tape. Now, hush your dulcet voice so we can explain the form.
"London, 1802" is a Petrarchan, a.k.a. Italian, sonnet. This form differs in rhyme scheme from the classic Shakespearean (English) sonnet. The Italian sonnet is divided up into an eight-line first section – an octave – that can be subdivided to two four-line mini-sections (quatrains) and following an ABBAABBA rhyme pattern. The second part of the poem is a sestet (that is, a six-line section) that can follow one of a few different rhyme schemes. The order that Wordsworth chooses to follow, CDDECE, is somewhat unconventional, but still within the realm of the Italian sonnet.
Whew! That was a lot of letters and numbers packed into two short paragraphs. To sum up, here's the rhyme scheme of the poem, just so we're clear:
Lines 1-8: octave – ABBAABBA
Lines 9-14: sestet – CDDECE