PU Tier is Partially Used tier. If you want to start battling like this, I'd LOVE to start battling a bit.
Here is a list of the PU Pokes up to Gen V. I am not 100% sure who is in Gen VI yet.
http://www.smogon.com/forums/threads/the-pu-viability-ranking-thread-read-post-112.3483077/ (http://www.smogon.com/forums/threads/the-pu-viability-ranking-thread-read-post-112.3483077/)