The tiers are:
Uber - Highest
OU - Second highest
UU - In Gen I the lowest
RU - Better than NU, worse than UU. Only existent in Gen V
NU - The bottom
LC - Levels 5's only, unevolved Pokemon. This does not mean they are bad Pokemon.
Limbo, BL, BL2, and NFE are unplayable tiers, however the Pokemon in them can be used in other tiers.