Thanks Lord Goomy :)
Magicknight like Sceptile said Pokémon Showdown is not official in the fact that it was not developed by Pokémon. Instead it was developed by Zarel who is an Administrator on the Website Smogon. Smogon is a community that specializes in competitive battling. They create tiers and rank a Pokémon in these tiers based on it's usage and how powerful is it, these tiers are designed to make the game more fair for players, as for example it's not very fair when an opponents Darkrai put all your Pokémon to sleep. Smogon and Pokémon Showdown are very technical as they have researched the game mechanics of the Pokémon Games and have adapted that in their battle simulator. They have many forums designed to help players play competitively and even have damage calculators to access the range of how much damage a Pokémon can do to another. They take into consideration entry hazards, weather etc. Smogon is one of the main Pokémon Websites that people use along with Bulbapedia, Serebii and of course the DB. They also collect data from players playing on Showdown and showcase their results to players.
