The blogosphere and writing chat rooms are bursting with questions about genre, manuscript length, and audience. Instead of reinventing the wheel I thought I'd link to some helpful articles that help break down these confusing issues.
From MiG Writers, here we have a comparison of MG versus YA.
From Suzie Townsend's Confessions of a Wandering Heart, here's some background on YA (and after what age you're technically reaching into the adult lit market).
Michelle McLean helps explain here the difference between paranormal, fantasy, and urban fantasy.
Guide to Literary Agents details what, exactly, is steampunk.
And on Query Tracker, agent Colleen Lindsay breaks down average word counts by genre.
I hope this helps!