Tiers are an attempt by players to classify characters in a game by their competitive ability. They can be found in any game with variable characters, including Pokémon and Super Smash Bros. The existence of tiers can be controversial as skill usually has more impact on the outcome of a match.
These are the offical Pokemon Tiers for Global Pokemon Union. Tiers made by me. Credit for ideas to Bulbapedia and Smogon.
Uber: Pokémon that are seen as too powerful for standard play. This could be due to massive stat total, insane movepool, or power to over-centralize the meta-game if they were to remain in OU (often, all 3 of these reasons apply).
Arceus
Darkrai
Deoxys
Deoxys-A
Deoxys-D
Deoxys-S
Dialga
Garchomp
Giratina
Giratina-O
Groudon
Ho-Oh
Kyogre
Latias
Latios
Lugia
Mew
Mewtwo
Palkia
Rayquaza
Shaymin-S
Wobbuffet
Wynaut
OU: Pokémon that are most often seen. They are generally recognized for this, their wide move pools and their high potential. The 40 most used Pokemon in Shoddy Battle are considered to be OU, However, there is 51 total OU's.
Salamence
Aerodactyl
Manaphy
Azelf
Blissey
Breloom
Bronzong
Celebi
Dragonite
Dusknoir
Electivire
Empoleon
Flygon
Forretress
Gengar
Gliscor
Gyarados
Heatran
Hippowdon
Infernape
Jirachi
Jolteon
Kingdra
Lucario
Machamp
Magnezone
Mamoswine
Metagross
Ninjask
Roserade
Rotom (all alternate forms)
Scizor
Skarmory
Smeargle
Snorlax
Starmie
Suicune
Swampert
Tentacruel
Togekiss
Tyranitar
Umbreon
Vaporeon
Weavile
Zapdos
BL: Pokémon that are recognized as being too powerful for UU play. Yet, these pokemon are a tad bit underwhelming when it comes to OU play. In some cases its just due to lack of field play.
Abomasnow
Cresselia
Crobat
Froslass
Gallade
Honchkrow
Porygon-Z
Raikou
Shaymin
Staraptor
Yanmega
UU: Pokémon that are not commonplace in a competitive environment due to more hindering flaws. They are generally lacking in potential, though several are still quite playable. With the increase in UU's, UU as a tier has gained some heavy ground in standing out as its own format of play. UU battles have become something of a common now.
Absol
Aggron
Alakazam
Altaria
Ambipom
Arcanine
Azumarill
Blastoise
Blaziken
Chansey
Claydol
Clefable
Cloyster
Donphan
Drapion
Dugtrio
Feraligatr
Hariyama
Heracross
Hitmonlee
Hitmontop
Houndoom
Kabutops
Kangaskhan
Lanturn
Leafeon
Ludicolo
Mesprit
Milotic
Mismagius
Moltres
Nidoking
Nidoqueen
Omastar
Poliwrath
Qwilfish
Regirock
Registeel
Rhyperior
Sceptile
Slowbro
Slowking
Spiritomb
Steelix
Swellow
Tangrowth
Torterra
Toxicroak
Ursaring
Uxie
Venusaur
Weezing
NU: Pokémon that are generally not seen at all due to being highly limited and unpopular. Their stats and movepools may and usually are very lacking compared to the higher tiers.
Ampharos
Arbok
Ariados
Armaldo
Articuno
Banette
Bastiodon
Beautifly
Beedrill
Bellossom
Bibarel
Butterfree
Cacturne
Camerupt
Carnivine
Castform
Charizard
Chatot
Cherrim
Chimecho
Clamperl
Corsola
Cradily
Crawdaunt
Delcatty
Delibird
Dewgong
Ditto
Dodrio
Drifblim
Dunsparce
Dustox
Electrode
Entei
Espeon
Exeggutor
Exploud
Farfetch'd
Fearow
Flareon
Floatzel
Furret
Gastrodon
Girafarig
Glaceon
Glalie
Golduck
Golem
Gorebyss
Granbull
Grumpig
Hitmonchan
Huntail
Hypno
Illumise
Jumpluff
Jynx
Kecleon
Kingler
Kricketune
Lapras
Ledian
Lickilicky
Linoone
Lopunny
Lumineon
Lunatone
Luvdisc
Luxray
Magcargo
Magmortar
Manectric
Mantine
Marowak
Masquerain
Mawile
Medicham
Meganium
Mightyena
Miltank
Minun
Mothim
Mr. Mime
Muk
Ninetales
Noctowl
Octillery
Pachirisu
Parasect
Pelipper
Persian
Phione
Pidgeot
Pinsir
Plusle
Politoed
Primeape
Probopass
Purugly
Quagsire
Raichu
Rampardos
Rapidash
Raticate
Regice
Regigigas
Relicanth
Sableye
Sandslash
Seaking
Seviper
Sharpedo
Shedinja
Shiftry
Shuckle
Skuntank
Slaking
Solrock
Spinda
Stantler
Sudowoodo
Sunflora
Swalot
Tauros
Torkoal
Tropius
Typhlosion
Unown
Venomoth
Vespiquen
Victreebel
Vileplume
Volbeat
Wailord
Walrein
Whiscash
Wigglytuff
Wormadam (All its Forms)
Xatu
Zangoose