OU, NU, and UU are tiers for online battling, most tiers are acronyms for something. There are quite a few more tiers also.
OU means OverUsed
>Overused, commonly abbreviated as "OU", refers to Pokémon that are most frequently used in standard play.
Some Pokemon in the OU tier are Charizard, Azumarill and Breloom
Uber means..... umm... Overpowered?
>The Uber tier is effectively a banlist for the OU tier, as "standard" battles include everything in the OU list and below. Pokémon in the Uber tier were deemed too powerful or destabilizing to the OU metagame to allow. Ubers, if allowed, cause extreme "centralization" of strategies—in other words, it is impossible or very difficult to counter them in a reasonable manner.
Some Pokemon in Ubers are Mega Lucario, Arceus, and Xerneas
UU means UnderUsed
>Underused, commonly referred to as "UU", consists of Pokémon that are not used enough to be in OU.
Some Pokemon in the UU tier are Aggron, Mamoswine and Whimsicott
BL means BorderLine
>Pokémon in Borderline, commonly referred to as "BL", are "too strong" for Underused, but are not sufficiently used in Overused battles. In essence, BL serves a ban list for Underused.
RU/LU means RarleyUsed/LittleUsed
>This tier is referred to as either "RU" or "LU". While the name of the tier varies, its "level" is equivalent. The tier consists of Pokémon that are not used enough to be in UU.
Some Pokemon in the RU tier are Abomasnow, Flygon and Drapion
BL2 means BorderLine2
>Pokémon in Borderline 2, commonly referred to as "BL 2", are "too strong" for Rarelyused/Littleused, but are not sufficiently used in Underused battles. In essence, BL 2 serves a ban list for Rarelyused/Littleused.
NU means NeverUsed
>Neverused, commonly referred to as "NU", consists of Pokémon that are not used enough to be in RU/LU. The name is not literal, stemming from the tiers of Generation I, where Neverused Pokémon had generally lower "competitive value" in comparison to other Pokémon. Some Pokémon that are pre-evolutions of Pokémon in higher tiers may be classified as NU.
Some Pokemon in the NU tier are Ferroseed, Pyroar and Samurott
BL3 means BorderLine3
>Pokémon in Borderline 3, commonly referred to as "BL 3", are "too strong" for Neverused, but are not sufficiently used in Rarelyused/Littleused. In essence, BL 3 serves a ban list for Neverused.
NFE means NotFullyEvolved
>This group is more commonly known as "NFE", and as the name implies, contains most Pokémon that have not yet reached their final evolution stage and do not have a high enough usage to appear in any tier. This group is not actually a tier, but rather a collection of Pokémon without significant usage even in the lowest tier.
Some Pokemon in NFE are Vibrava, Charmeleon and Croconaw
>Little Cup is an entirely separate battle style where only the Pokémon that are the lowest evolutionary stage in their family are allowed, like Bulbasaur and Mienfoo. Pokémon that do not evolve at all, such as Luvdisc, are not allowed in the tier. The moves Sonic Boom and Dragon Rage are also prohibited. All Pokémon must be level 5 or lower to compete.
Some Pokemon in Little Cup are Onix, Pawniard and Squirtle
Hope I helped!