Ahhh, the eternal question. Like many things (such as honesty or friendship) it is difficult to define but "you know it when you see it."
I recently came across a few short lines by American poet and teacher Marvin Bell who has tried to 'define' what makes something poetry:
Prose is prose because of what it includes; poetry is poetry because of what it leaves out.
What they say "there are no words for"--that's what poetry is for. Poetry uses words to go beyond words.