"Stopping by Woods on a Snowy Evening" is a lyric poem. A lyric poem is one that expresses personal emotion, often using the first person "I" voice to express what the poem's speaker is feeling.
Frost's poem begins with such a first-person voice. In the opening line the speaker states,
Whose woods these are I think I know.
Like many lyric poems, "Stopping by Woods on a Snowy Evening" has a narrative element. The poem begins in media res, or the middle of the action. The speaker is on a journey and has decided to stop to appreciate the beauty of the woods as the white snow falls amid the trees. While the speaker is quiet and understated, his emotions come through most fully in the final stanza, in which duty tugs against pleasure:
The woods are lovely, dark and deep,
But I have promises to keep
Unlike an occasional poem, which might celebrate the coronation or death of a king or some other important public event, this poem is a lyric because it attends a private emotion. Nobody sees the speaker stop and appreciate the stillness and beauty of the woods. Yet almost all readers can connect with the experience of suddenly stopping and noticing the profound loveliness of the world around them.
The poem is also a lyric one in that it is short. Unlike an epic poem, like, for example, the Iliad, which is long and tells the story of a highly public event, this poem captures a fleeting moment and a fleeting emotion.
Formally, the poem is composed of four stanzas of iambic tetrameter lines that follow an AABA rhyme scheme. Somewhat like Dante's terza rima form, the B rhyme in any given stanza dictates the A rhyme in the ensuing stanza. The scope and style of the poem are generally reflective of the lyric mode, though Frost's particular chain-rhyme structure is not common.