Pokémon features hundreds of species available for use, all varying in
their base stats and moveset potential. There are currently six tiers(Uber, OU, BL, UU, RU, NU, and NFE)
with various fansites individually deciding the Pokémon assumed to be
in those tiers, most notably Smogon. Pokémon tier lists have received
much criticism, because there is no "official" tier list for Pokémon.
However, official tournaments often have ban lists. In "standard"
battles, any Pokémon that is not in the Uber tier is eligible for use.
However, it is common for some battles to maintain use in one tier
only.
http://bulbapedia.bulbagarden.net/wiki/Tiers
They are tiers.
