The poem explores the effect of nature's beauty on the human psyche. A one sentence summary might be: A man on horseback stops at the edge of the woods to watch the beauty of falling snow before succumbing to his obligations and resuming his journey.
The poem employs a wide range of poetic techniques, including--but not limited to--the following: rhyme, meter, symbolism, parallelism, and alliteration.
Please go to this link on eNotes: http://www.enotes.com/stopping-by-woods-snowy-evening/style
It specifically discusses the poem's rhyme scheme and other literary devices. You will also find helpful information about the poem's themes, background, etc.