The time of battles has begun edition
Chat room: http://play.pokemonshowdown.com/cpg - circlejerk here

Introduction to Competitive Pokémon: http://www.smogon.com/dp/articles/intro_comp_pokemon
Tiers Explained: http://www.smogon.com/bw/articles/bw_tiers
Clauses and Banlists: http://www.smogon.com/bw/banlist/
StrategyDex & Movelists: http://www.smogon.com/bw/pokemon/


Online Pokémon Simulators:

Pokémon Showdown: http://www.pokemonshowdown.com
Pokémon General: http://www.pokemon-online.eu/
Pokémon TCG Online: http://www.pokemontcg.com/

Official Website: http://www.pokemon.com/us/


Other Informative Websites:

Serebii: http://www.serebii.net/index2.shtml
Bulbapedia: http://bulbapedia.bulbagarden.net/wiki/Main_Page
Veekun: http://veekun.com/

First posts # determines the tier
1-Uber
2-OU
3-UU
4-RU
5-NU
6-Hackmons
7-Ability Exchange
8-STABmons
9-Old gens
0-Random Battle