"Grass" is written in free verse, which means that it doesn't have a regular rhyme scheme or meter. Carl wasn't interested in writing in forms like sonnets, villanelles, or even haikus. Our poet aimed to write in natural, straightforward language, and didn't want to be weighed down by lots of poetry rules.
That being said, "Grass" does have one really interesting formal feature: repetition. Sandburg loves repeating lines like "I am the grass" and "let me work" for emphasis. By repeating these lines multiple times, Sandburg sears them into our brains, and we get the feeling that the grass's work is never done—it's been doing its job since the early nineteenth century.
And when repetition occurs at the beginning of lines—such as the "And pile them high" lines (4-5)—we've got a fancy Greek name for that: anaphora. We bet that Carl would have hated using fancy poetry terms to describe his work, but he's just such a master of anaphora that we've gotta share the terminology love.
It's a good choice for this poem, because the repetition of form here really underscores the relentless, and predictable, work that the grass undertakes. Its job is the same every time: go out and erase the horrors of history. Luckily, we have this poem (and its form) to draw our attention to that fact so that we can try and do something about it.