OU and UU are both commonly-used acronyms for the name of two Pokemon tiers, Overused and Underused respectively. It is kind of hard to explain what they are without explaining other thing first though, so yeah.
The most popular Pokemon tiers are created by Smogon.com, a competitive Pokemon battling website that has allocated tier for each Pokemon in each Gen (they're still working on X and Y though). OU and UU are the second and third highest tiers on the list of tiers, only surpassed by Ubers which is essentially OU's ban list anyway. OU is Smogon's most popular tier to play in since Pokemon in tiers above the tier you play in are banned, and since the most OP legendary Pokemon of the game are stuck in Ubers, OU is a good balance to the best Pokemon that game has to offer outside of Zekrom and Lugia stuff.
UU is the tier following OU. Usually, Pokemon in UU are good Pokemon that might have been in OU for a previous Generation, but suffer from a number of shortcomings that hold them back from OU. Usually this include move pool issues, mediocre typings, stuff like that.
An unplayable called BL (Borderline) is where a number of Pokemon that've been banned from UU reside. These Pokemon are only allowed in OU and Ubers. Basically, if Smogon sees a Pokemon as making a certain tier unbalanced, they hold a suspect test (which is basically a vote with senior players of UU). If the majority of people vote one way or another, the suspected Pokemon stays or leaves for BL. This goes for every tier - banned RU Pokemon go to BL2, and banned NU Pokemon go to BL3. Banned OU Pokemon however, go straight to Ubers. Ubers, as I mentioned before, is essentially OU's ban list.
All the tiers and their Pokemon are listed here: