Williams was one of the pioneers of free verse: poetry with no regular rhyme or rhythm. Of course, the problem with a term like "free verse" is that it's defined by what it lacks, not what it has. So what kind of form does this poem have? Let's start with the obvious: it's short. At thirteen lines and 31 words, that's less than three words per line. The whole thing is only a single sentence broken up into lines. There are no punctuation marks at the end of any of the lines, so they are all enjambed – each line carries over into the next without a pause. Nonetheless, when you read it aloud, you probably add small pauses rather than barreling through the whole sentence at once.
With its length and attention to highly focused images, the poem resembles certain forms of Asian poetry. Asian poetry was a central influence on the Imagist movement, with which Williams's early poetry is often associated. Williams even translated some Chinese poems, which you can read here.