poetry vs prose

I wonder if prose really is freer than poetry (of course recognizing that the line between the two is far from clear). For the most part we talk (and maybe even think, though I'm not convinced yet) in prose, so is poetry a way to free us from the every day? Then again poetry is by definition constrained (even free verse). Is that why prose poetry is growing in popularity? How does one define prose? It seems to be viewed as just unmarked writing, but that makes it seem like the pure form that all others are to be compared against. I can't accept that.