A Tier are Pokemon classifications, all varying in their base stats
and moveset potential. There are currently seven tiers with various
fansites individually deciding the Pokémon assumed to be in those
tiers. 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.
The 7 Tiers are Uber, OverUsed, Borderline, Under Used, Rarely Used,
Neverused, and Not Fully Evolved. Hope this helps...