Blank Verse (Mostly Unrhymed, Iambic Pentameter)
Frost writes this poem in blank verse, meaning that it doesn't rhyme (sad), but that it does have interesting structure stuff going on. The poem loosely follows an iambic pentameter structure. But what the heck does that mean? "Iambic" refers to the pattern of stresses in the line. An "iamb" is an unstressed syllable, followed by a stressed syllable: da-DUM. "Pentameter" means that there are five ("penta") iambs in the line: da-DUM da-DUM da-DUM da-DUM da-DUM. Easy, right?
Although Frost wrote some formal and conservative verse, he's not known for that kind of poetry. Rather, Frost earned critical and popular attention for his verse written in blank and free verse. He liked to imitate the sound of regular or rural speech. English has a tendency to fall into the rhythm of iambs, but occasionally throws in an anapest (which is two unstressed syllables followed by a stressed syllable, da-da-DUM) at the beginning of a sentence or a dactyl (which is a stressed syllable followed by two unstressed, DUM-da-da). Frost's verse, like English, is irregular but works off of common patterns.