i don't mean to sound stupid but what exactly is a UN and a OU pokemon.
Not a stupid question. In competitive battling, there are tiers that help divide Pokémon into certain brackets based on how frequent they are used and how useful they are in competitive battling. There are more than just "NU" (I believe that's what you mean) and "OU" as listed below:
Uber: Typically Pokémon banned from being used in other tiers. They usually consist of legendaries but not always.
Over-Used (OU): The most popular tier in the metagame. This consists of Pokémon you're more than likely to see used in the metagame.
Borderline (BL): Pokémon that exceed the expectations for the tier Under-Used, but doesn't exactly fit in the OU bracket.
Under-Used (UU): Pokémon that aren't used very often in competitive battles and you're not likely to see them used most often in the OU tier.
Never-Used (NU): The title isn't to be taken literal, but these are Pokémon that you're not typically going to see being used very much or at all.
Not Fully Evolved (NFE): I think this one speaks for itself.
I think that covers most of it. I haven't been in the competitive scene for such a long while honestly.
Edit: Got my butt whopped by the above posters. But you get the idea now!
I'm expecting X and Y to shake up the metagame scene a bit. At least I hope so, maybe I might get back into it again...or maybe not. But hey, you never know!