I know this was already answered, but I am adding in links to lists of the Pokemon.
Ubers: Every Pokemon are allowed in ubers and the clauses are: Swagger Clause: Swagger is banned,Sleep Clause Mod: Limit one foe put to sleep, Species Clause: Limit one of each Pokémon, Moody Clause: Moody is banned, OHKO Clause: OHKO moves are banned, Endless Battle Clause: Forcing endless battles is banned, HP Percentage Mod: HP is shown in percentages. [Here][1] is all the uber Pokemon.
OU: All Pokemon EXCEPT uber are allowed in OU. Here is all the clauses: Swagger Clause: Swagger is banned, Baton Pass Clause: Limit one Pokémon knowing Baton Pass, Sleep Clause Mod: Limit one foe put to sleep, Species Clause: Limit one of each Pokémon, OHKO Clause: OHKO moves are banned, Moody Clause: Moody is banned, Evasion Moves Clause: Evasion moves are banned, Endless Battle Clause: Forcing endless battles is banned, HP Percentage Mod: HP is shown in percentages. [Here][2] is all the OU Pokemon.
BL: These Pokemon are too strong for UU, but not quite OU. [Here][3] is a list of BL Pokemon.
UU: All Pokemon EXCEPT Pokemon in Uber or OU. All the clauses are the same from now on. [Here][4] is all the UU Pokemon.
BL2: These are Borderline 2 Pokemon. These Pokemon are too strong for RU but not quite UU. [Here][5] is a list of BL2 Pokemon.
RU: All Pokemon in RU, BL3, NU, LC and NFE can be used in RU. [Here][6] is a list of all RU Pokemon.
BL3: These Pokemon are too good for NU, but not quite RU. [Here][7] is a list of the 2 BL3 Pokemon.
NU: All Pokemon in NU, LC and NFE can be used in NU. [Here][8] is the list.
NFE: These are Not Fully Evolved Pokemon. All Pokemon that are not fully evolved can be used in it. [Here][9] is a list.
LC: These are the first evolution of 3 evolution Pokemon. All Pokemon in this tier are level 1-5. [Here][10] is a list of the Pokemon.
Ok, I'm done. I hope this helped!
[1]: http://pokemon.neoseeker.com/wiki/Tier_listings#Uber
[2]: http://pokemon.neoseeker.com/wiki/Tier_listings#Overused_.28OU.29
[3]: http://pokemon.neoseeker.com/wiki/Tier_listings#Borderline_.28BL.29
[4]: http://pokemon.neoseeker.com/wiki/Tier_listings#Underused_.28UU.29
[5]: http://pokemon.neoseeker.com/wiki/Tier_listings#Borderline_2_.28BL2.29
[6]: http://pokemon.neoseeker.com/wiki/Tier_listings#Rarelyused_.28RU.29
[7]: http://pokemon.neoseeker.com/wiki/Tier_listings#Borderline_3_.28BL3.29
[8]: http://pokemon.neoseeker.com/wiki/Tier_listings#Never_Used_.28NU.29
[9]: http://pokemon.neoseeker.com/wiki/Tier_listings#Not_Fully_Evolved_.28NFE.29
[10]: http://pokemon.neoseeker.com/wiki/Tier_listings#Little_Cup_.28LC.29
