Like when i read the comments for the RBY movesets sometimes it talks about UU (w/e that is) Id like to know o.o

Underused. As in not used very often.

OU is overused as in people use it a lot.

NU is never used, This is mostly for basic pokemon and sunflora.

Poe stated it correctly, but to expand, UU, OU, NU, and Uber are all tiers in the competitive battling metagame. All pokemon within a Tier are assumed to be roughly equal, in terms of movesets and stat spreads, so that a match between two teams of UU pokemon should be fairly equal.