I guess it helps to have a big imagination to take in enough poetry so you can make up your own mind about what it is. I feel like every time I read a new poem, the dimensions of my imagination expand a little more in some direction. There are poems that string themselves out into novel lengths and become epic tales. If poetry is verse and the Bible is written in verse, perhaps we can think of the Bible as one long poem. Armed with an all-purpose poetic license, the poet can disobey rules of language, ignore punctuation and spelling, toss words all over a page, cluster lines into islands of words, and let loose a hurricane of emotion.
Poems can be about anything.
I've written poems about writing poems. I've written poems about Wal-Mart and Y2K, waitresses and waterfalls, magical beasts and many a lost love. There's slam poetry and southern fried poetry, spiritual poetry and sexy poetry. There are lewd limericks and Shakespearean sonnets. There's rap and rondels, odes and elegies, ballads and blank verse, villanelles and heroic couplets.