OU, UU, RU, and NU are all determined by usage. Ubers, BL, and BL2 are made up of things that overcentralize the tiers directly below them. --- 4670791 Si || Everyone dies. It is how one lives that matters.
to expand a little bit, often a pokemon's stats will affect how much it is used - thus, higher BST pokes tend to be in higher tiers. there are exceptions.
also, pokes may be bumped up/down a tier if they're overly-prevalent, lack many true counters, or are simply too easy to exploit in their current tiers. i.e., Excadrill or SB Blaziken --- The weird tree attacked!
Most tiers (OU, UU, RU, and NU) are based on usage. Almost all Pokémon are in these tiers. If everyone is using Scizor, then Scizor will end up in OU, for example. Tiers (when they originated) essentially started with all the Pokémon in a pool, and some were used more often, which became OU and UU to an extent, and the others drifted down into RU and NU. Later, they banned certain Pokémon from each of these tiers; Uber (the banlist for OU) is the only one played as its own tier. --- For those of you starting topics about the PS4, there's a PS4 board: http://www.gamefaqs.com/boards/691087-playstation-4