If the lines of whatever you're reading stop before the end of the paper or screen, congratulations, you're reading poetry. It's really that simple.
Poetry is everything
that's not prose,
and often means
the writer really loves
to hit return. A lot.
But alas, now you're not, because this line's going all the way to the end of your screen, baby, and then some. You know how prose do.
Some folks will tell you that poetry's full of metaphors and figurative language. They're wrong. Prose can be chock full of those things, too, and don't you forget it. Plus, some poems have none of these things.
And some folks will say that some prosey pieces of writing are in fact poems (prose poems, to be exact), because of their emphasis on sound and brevity. They're not wrong. What can we say? It's complicated.