In the competitive system, there are certain tiers where Pokémon are used depending on their stats. These tiers were designed for competitive players to create teams with equal Pokémon, so you'll not see a Legendary Pokémon facing against an Accelgor in the lowest tiers.  So, the highest tier is Uber, where you can play with the strongest legendaries and some other Pokémon, then OU (Over Used), UU (Under Used), RU (Rarely Used), NU (Never Used), and lastly, PU, which is the lowest tier where you'll even see players with untiered Pokémon.  You'll not see a Uber Pokémon against an NU Pokémon in NU. Still, you can see NU Pokémon in Ubers, depending on the trainer.