From Bulbapedia, the community-driven Pokémon encyclopedia.
Tiers are an attempt by players to classify characters in a game by their competitive ability. They can be found in any game with variable characters, including Pokémon and Super Smash Bros. The existence of tiers can be controversial as skill is usually has more impact on the outcome of a match. As such, tiers should be seen as a classification of each character's competitive potential at the height of the metagame.
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.
Most battles are conducted using every tier except Uber. However, it is common for some battles to maintain use in one tier only. Usually Under-Used only, or Never-Used only.
The Uber tier is not designed to be balanced, its purpose is to be 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 all, or only, legendary Pokémon. While it is true that a majority of the accepted Ubers are indeed legendary, some are not and many legendary Pokemon 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 an almost free kill with Counter or Mirror Coat which let it reflect most moves back. Tickle+Encore allows Wobbuffet to bring a slow Pokemon using a setup move down to -6 Defense with almost no risk, ready for a Pursuit user to switch in and KO. Wobbuffet's pre-evolution, Wynaut, is sometimes banned for similar reasons. Garchomp is another non-legendary that some fans consider for the list due to its high stats, brilliant movepool, and great typing.
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.
Another notably interesting exception is Ninjask. Its stats are generally very poor outside of its Speed, but due to large supportive movepool and Speed Boost ability, it is arguably the best user of Baton Pass allowing it to pass major stat boosts to other Pokémon on the player's team.
Examples of common OU Pokémon are the aforementioned Skarmory, Ninjask, and Magnezone, as well as many other Pokémon including, but not limited to, Blissey, Scizor, and and almost all of the pseudo-legendaries like Tyranitar.
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. They are classified as BL because they generally do not have the capacity to compete in solid OU consistently due mostly to stats, weak movepools, or metagame shifts that makes the niche of the Pokémon less needed.
The Pokémon placed in BL change with the metagame, but some examples include Crobat, Staraptor, or Shaymin.
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.
Examples of UU include Swellow, Clefable, or Toxicroak.
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. 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.
The most notable examples are the aforementioned novelty Pokémon, like Pachirisu or Chatot, and early, easily obtained Pokémon, like Raticate or Ledian, but can also include moderately powerful Pokémon like Magcargo, Exploud, or Kingler. Kingler for example has a massive 130 base Attack, but an extremely poor and easily exploitable base Special Defense of only 50, as well as poor Speed of 75, meaning its use is very diminished, especially with the abundance of different Template:Type2 Pokémon..
Not Fully Evolved
This tier is more commonly known as "NFE" and as the name implies, contains almost ever Pokémon that has not reached its final evolution. As such, the are rarely, if ever used in competitive play, with very few exceptions.