In Layman's Terms

Just to clarify, tiers are basically used to separate out pokemon that are commonly used during competitive play.  Tiers are used to assist people when building their team so that they're aware of what the most popular pokemon are (OU) and are able to accommodate appropriate counters into their team.  A tiers list is not meant to suggest that some pokemon are better than others, it is to be used solely as a reflection of the current team trends that exist in the metagame.

For instance, suppose you want to consider putting Ledian on your team.  Take a quick look at the tier list and you'll see how popular Rock, Ice, and Fire type are; Tyranitar, Weavile, and Infernape for instance.  In addition to that it should show you that a large number of these pokemon use Steal Rock, such as Hippowodon, Bronzong, and Swampert.  When you take such things into consideration you may wish to rethink putting Ledian on your team, or better yet, if you do choose to put it on your team then specify to your opponents that you'd like to play with pokemon only in a certain tier.

As I mentioned before though, the most important thing to be doing when looking a tier list is to make sure your new team will be able to effectively counter certain popular pokemon, so that your team will function well in a competitive environment; such as putting on a counter with levitate so that you'll be able to fight pokemon such as Garchomp which is so widely used (thus it's listed in the OU tier), or that you'll instead consider putting on an ice type so as to take advantage of Garchomp's 4x weakness to Ice. 

Credits to Smogon for the tier list and SkyUppercut60 for help with the introduction.

{tab=Uber} 

Arceus
Darkrai
Deoxys
Deoxys-A
Deoxys-D
Dialga
Giratina
Groudon
Ho-Oh
Kyogre
Latias
Latios
Lugia
Manaphy
Mew
MewTwo
Palkia
Rayquaza 

{tab=OverUsed (OU)}

Abomasnow
Azelf
Blissey
Breloom
Bronzong
Celebi
Cresselia
Deoxys-S
Donphan
Dragonite
Dugtrio
Dusknoir
Electivire
Forretress
Gallade
Garchomp
Gengar
Gliscor
Gyarados
Heatran
Heracross
Hippowdon
Infernape
Jirachi
Jolteon
Kingdra
Lucario
Machamp
Magnezone
Mamoswine
Metagross
Milotic
Ninjask
Porygon-Z
Salamence
Scizor
Skarmory
Snorlax
Starmie
Suicune
Swampert
Tentacruel
Togekiss
Tyranitar
Vaporeon
Weavile
Wobbuffet
Yanmega
Zapdos

{tab=Borderline (BL)} 

Aerodactyl
Alakazam
Ambipom
Arcanine
Articuno
Azumarill
Blaziken
Charizard
Crobat
Empoleon
Entei
Espeon
Exeggutor
Feraligatr
Floatzel
Flygon
Gardevoir
Hariyama
Honchkrow
Houndoom
Lickilicky
Ludicolo
Magmortar
Marowak
Medicham
Mesprit
Miltank
Mismagius
Moltres
Pinsir
Porygon2
Raikou
Rampardos
Regice
Regigigas
Regirock
Registeel
Rhyperior
Roserade
Sceptile
Shaymin
Shedinja
Slaking
Slowbro
Slowking
Smeargle
Spiritomb
Staraptor
Tangrowth
Tauros
Torterra
Typhlosion
Umbreon
Ursaring
Uxie
Venusaur
Weezing
Zangoose

{tab=Underused(UU)} 

 Absol      
Aggron     
Altaria     
Ampharos
Arbok     
Ariados     
Armaldo     
Banette     
Bastiodon     
Beautifly     
Beedrill     
Bellossom     
Bibarel     
Blastoise     
Butterfree     
Cacturne     
Camerupt     
Carnivine     
Castform     
Chatot     
Cherrim     
Chimecho     
Clamperl     
Claydol     
Clefable     
Cloyster     
Corsola     
Cradily     
Crawdaunt     
Delcatty     
Delibird     
Dewgong     
Ditto
Dodrio     
Drapion     
Drifblim     
Dunsparce     
Dustox     
Electrode     
Exploud     
Farfetch'd     
Fearow     
Flareon     
Froslass     
Furret     
Gastrodon     
Girafarig     
Glaceon     
Glalie     
Golduck     
Golem     
Gorebyss     
Granbull     
Grumpig     
Hitmonchan     
Hitmonlee     
Hitmontop     
Huntail     
Hypno     
Illumise     
Jumpluff     
Jynx     
Kabutops     
Kangaskhan     
Kecleon     
Kingler     
Kricketune     
Lanturn     
Lapras     
Leafeon     
Ledian
Linoone     
Lopunny     
Lumineon     
Lunatone     
Luvdisc     
Luxray     
Magcargo     
Manectric     
Mantine     
Masquerain     
Mawile     
Meganium     
Mightyena     
Minun     
Mothim          
Mr. Mime     
Muk     
Nidoking     
Nidoqueen     
Ninetales     
Noctowl     
Octillery     
Omastar     
Pachirisu     
Parasect     
Pelipper     
Persian     
Phione     
Pidgeot     
Pikachu     
Plusle         
Politoed     
Poliwrath     
Primeape     
Probopass     
Purugly     
Quagsire     
Qwilfish     
Raichu     
Rapidash     
Raticate     
Relicanth     
Rotom     
Sableye     
Sandslash     
Scyther     
Seaking     
Seviper     
Sharpedo     
Shiftry     
Shuckle     
Skuntank     
Solrock     
Spinda     
Stantler     
Steelix     
Sudowoodo     
Sunflora     
Swalot
Swellow     
Torkoal     
Toxicroak     
Trapinch     
Tropius     
Unown     
Venomoth     
Vespiquen     
Victreebel     
Vigoroth     
Vileplume     
Volbeat     
Wailord     
Walrein     
Whiscash     
Wigglytuff     
Wormadam     
Wormadam-G     
Wormadam-S     
Xatu