It's poetry. It's prose. It's two great tastes in one. This is a poem, but it's written in a block, without the broken lines or stanza breaks present in most poems. For that reason, it's considered a prose poem.
When you look at it on the page, it creates a visual column, not the usual in-out saw teeth of a poem that places emphasis on the last words of the line. Instead, you have lines that end on totally who-cares words like "the" and "on." And as for meter and rhyme, two of the usual suspects found in poems, there's really none to be found here. The lines are purposely flat and un-cadenced, in most cases.
So what's the deal with this very atypical poem? No conventional rhyme or structure—not even a typical set of line breaks. Sheesh. It's as if Forché deliberately tries to keep the poem from sounding, or even looking, pretty. Written this way, this poem can do double duty. It's almost as if Forché wants this poem to have the status of a news dispatch. Think about newspaper articles, how concise they are, how they try to get across the 5 Ws (who, what, when, where, why), how objective they're supposed to be. The choices Forché makes with the form and meter of this poem suggest that she wants to take herself, or at least her speaker, out of the story, so that it can speak for itself.
What's left, then, is a kind of "just the facts, ma'am" style of reporting, which makes the effect of the poem all-the-more striking. There's no need to dress up a grocery bag full of severed ears with any sort of fancy rhythms or line breaks. That's the kind of horror that can speak for itself. And in this prose poem, that's exactly what happens.