NU teams are teams made of Pokemon in the NU tier.
OU teams are teams made of Pokemon in the OU tier, or lower.
Same for UU and RU
They stand for Neverused Underused Overused and Rarelyused.
They basically stand for their names. Pokemon you see a lot in the competitive battling community with good stats will be put into OU, Pokemon with not so great stats and minimum to usage at all in the competitive battling community will be put in the NU/RU/UU teams. It really depends on how they match up against other Pokemon in that tier.
NU teams have NU Pokemon. OU has OU pokmeon. UU has UU Pokemon. The only difference is the Pokemon and a few other things like bans.
Ban list differences