The most popular and most used tiers belong to Smogon. Their tiers can be found here. However, there are other tiers as well, as tiers are unofficial and are fan-made. The DB once had tiers as well. Each Pokemon are grouped in tiers for their usage and reliability in general, to prevent battles with Magikarp vs. Arceus. The tiers are updated every here and then, but other than that tiers never change until another Generation is released, which is what causes metagame gameplay changes.
So basically there are many opinions and different tier methods, so it's not really correct to say Smogon's tiers are official.