Currently, there are only 3 tiers out, Ubers, OU, UU Beta (this is excluding LC and doubles).
What Smogon has done was taken the Pokemon considered to OP and placed them into Ubers, then took the Pokemon most used by competitive players and placed them into OU, and threw in 3 Limbo tiers, which basically makes up the UU Beta tier.
Pokemon considered Uber are unallowed in the OU tier, here's the list:
All forms of Arceus, Blazekin, Darkrai, Deoxys, Deoxys-A, Dialga, Giratina, Giratina-O, Groudon, Ho-Oh, Kyogre, Kyurem-W, Lugia, Mewtwo, Palkia, Rayquaza, Reshiram, Shaymin-S, Xerneas, Yveltal, Zekrom.
Pokemon considered in the OU tier are unallowed in UU Beta. Here is the tier list:
Aegislash, Alakazam, Azumarill, Blissey, Breloom, Charizard, Clefable, Cloyster, Conkeldurr, Donphan, Dragonite, Espeon, Excadrill, Ferrothorn, Forretress, Galvantula, Garchomp, Genesect, Gengar, Gliscor, Goodra, Greninja, Gyarados, Heatran, Infernape, Jirachi, Kangaskhan, Klefki, Landorus, Landorus-Therian, Latios, Lucario, Mamoswine, Mandibuzz, Mawile, Pinsir, Rotom-Wash, Sableye, Salamence, Scizor, Skarmory, Smeargle, Starmie, Sylveon, Talonflame, Tentacruel, Terrakion, Thundurus, Togekiss, Trevenant, Tyranitar, Venusaur, Volcarona, Dialga, Giratina, Giratina-O, Groudon, Ho-Oh, Kyogre, Kyurem-W, Lugia, Mewtwo, Palkia, Rayquaza, Reshiram, Shaymin-S, Xerneas, Yveltal, Zekrom.
Then you have Pokemon placed into the BL tier who are also banned in UU, but aren't considered OU:
Manaphy and Kyurem-Black
Remember that OU and UU are based off of usage and that these tiers are subject to change once the find something OP enough to be bumped out of UU and into OU. In a few moths, the RY tier will be put in place which will be based off of the usage stats in UU.