Tiers in Pokémon
Pokémon features hundreds of species available for use, all varying in their base statistics and moveset potential. There are currently six tiers with various fansites individually deciding the Pokémon assumed to be in those tiers, most notably Smogon. The Pokémon tier list has received much criticism, because there is no "official" tier list for Pokémon.
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.
Uber
The Uber tier is a banlist for OU and the other tiers, however this does not mean it is not, or cannot be played. Pokémon in the Uber tier are deemed too powerful or centralizing to be used fairly among other Pokémon. A common misconception, especially among players new to competitive battling, is that Uber tier contains only legendary Pokémon. While it is true that a majority of the accepted Ubers are indeed legendary, some are not and many legendary Pokémon are far from being Uber.
While most Pokémon in the Uber tier are there for their ability to power through teams or be defensive behemoths, Wobbuffet is included primarily because of its Shadow Tag ability, along with Encore, which lets it trap any opponent Pokémon and lock them into any move, giving a free turn of set-up for any Pokémon, or giving a "free" kill with Counter or Mirror Coat. Wobbuffet's pre-evolution, Wynaut, is sometimes banned for similar reasons. Garchomp is another non-legendary that is in the list the list due to its high stats, brilliant movepool, and great typing.
Over-Used
Over-used, commonly abbreviated as "OU", refers to Pokémon that are most commonly seen in standard play. It is a misconception that all OU Pokémon are the most powerful in the game. OU is a term based on usage; however, there is a distinct correlation between usage and usefulness in the system.
There are exceptions to this such as Magneton in Generation III, where it was considered an OU Pokémon merely because it was used primarily as a counter for Skarmory; a Pokémon commonly seen due to its high defense, low weaknesses, and stall-based movepool. The introduction of Magnezone, an evolution of Magneton, solidified its place in OU, due to increased stats, and a broader movepool.
Borderline
Pokémon in Borderline, commonly referred to as "BL", are considered by some to be too strong for Under-Used, but may be found in some OU battles. A Pokémon is placed in BL only if it is too strong for the UU tier; its performance in OU has nothing to do with its status.
Under-Used
Under-Used, commonly referred to as "UU", designates Pokémon whose use is limited as they are not strong enough for general OU competition, usually due to sub-standard stats, movepools, easily exploitable weaknesses, or the existence of a stronger Pokémon that do nearly the same job as the UU Pokémon, making players use the stronger Pokémon instead of the UU Pokémon, giving it little use. UU Pokémon can, and are, used in OU battle, but are usually recommended by some solely for UU-type battles.
Never-Used
As the name implies, Never-Used, commonly referred to as "NU", is for Pokémon that have problems competing even in UU due to dismal stats and/or movepools, and thus are extremely rare to be used in battles. This is where most novelty Pokémon reside in tiers as well as Pokémon found early in a player's journey. They are recommended by most only for use in NU only battles. However, most NU Pokémon are very common among contest coordinators.
Not Fully Evolved
This tier is more commonly known as "NFE", and as the name implies, contains almost every Pokémon that has not yet reached its final evolution. Some NFE Pokémon, such as Rhydon are used in the lower tiers, as their stats are on par with some of the weaker evolved Pokémon. Also, the abilities of Hippopotas and Snover are the only way to set up permanent weather in the lower tiers, as their evolved forms are banned.
