Poetry is language rendered musically. Its long history is rooted in the prehistoric oral tradition of the first humans, evolves past the advent of writing, and continues to evolve into the present. It takes on many forms, from simple rhyming couplets to complex free verse. A much misunderstood art form, poetry nevertheless continues to inspire millions of readers and listeners in every culture.
Poetry contains both literary and musical features. On the literary side, poetry can use any combination of metaphor, simile, imagery, symbolism, allegory, even narrative or storyline. Denotation (direct meaning) and connotation (indirect meaning) help to further sculpt the literary meaning or sense of a poem. On the musical side, poetry uses rhyme, meter, assonance (similar vowels) and alliteration (similar consonants) to strike an aural mood or tone. Poetry can be ironic or humorous, attractive or ugly, mystical or mundane.
Poetry has taken on many forms and has been defined in many different ways over thousands of years among the world's cultures. The earliest known recorded poetry comes from ancient Mesopotamia (c. 3000 BC), inscribed in cuneiform on stone tablets used for religious purposes. Chinese poetry found its canonization in 1000 BC. The poetry of the ancient Greeks closely followed, epitomized by the epic novels of Homer (c. 800 BC). Dante's Divine Comedy, penned in the 14th century, is considered the crowning literary achievement of medieval Europe, while the Enlightenment and the Romantic Period (epitomized by Samuel Taylor Coleridge's "Rime of the Ancient Mariner") continued the European tradition. The 20th century saw the rise of post-modernism, which essentially did away with all the traditional rules. Contemporaneous poetry takes on virtually all of these aesthetic forms and more.
Poetry has the power to move hearts, win minds, and inspire people to action. Take Martin Luther King's "I Have a Dream" speech as an example. His use of theme, tone and rhythm in the speech unified millions of Americans in the ongoing struggle for universal civil rights. Or take the "Fifth of November" nursery rhyme, written in response to Guy Fawkes' 1606 attempt to bomb the English Parliament. To this day most Britons can recite the catchy rhyme from memory--evidence that acts of terrorism are not easily forgiven. Going further back in history, the Psalms of David, which are essentially a collection of poetic song lyrics, have been inspiring the faith of Hebrew and Christian peoples for many thousands of years.
When approaching a poem, listen. What is the poem saying and how is it being said? Do you notice any interesting turns of phrase which could have been stated more blandly? What types of musical elements do you hear? If there is no obvious rhyme scheme or rhythmic pattern, can you detect assonance or alliteration? When one of those elements is repeated, you can bet the poet did it on purpose, as intentionality is the hallmark of a pleasing poem. Consider also the context in which the poem was written. Oftentimes if you know something about the poet herself--how she grew up or what she does for a living, for example--you will be better prepared to catch subtle allusions where otherwise you might only have found obscurity. Just don't be surprised if a poem hits you with all the blunt force of a stone tablet; not all poetry is parchment-delicate.
How does one distinguish between poetry and literary prose? Both make use of language to convey mood, sense and imagery. Both must follow the basic rules of language so that the reader or listener can comprehend the words. The main difference between poetry and prose, however, is music. While prose chiefly concerns itself with the cerebral experiences of meaning and narrative, poetry tends to appeal to the human ear itself. In modern poetry, rhyme, rhythm, tone and cadence are highlighted, and narrative is often de-emphasized. As such, poetry is more of a physical experience than prose. Yes, elements of each appear in the other, often blending until they are indistinguishable. But if you are ever forced to explain the difference, tell them it's the music.