Don't Know Anything About Pokemon Tiers ? Don't Worry , We'll Help You !
Over-Used, commonly abbreviated as "OU", refers to Pokémon that are most frequently used in Standard Play.
Being designated OU does not have to do with a Pokémon's "power" or "ability". OU, as with all tiers, is defined based on usage. It may change by generation or even within a generation. For instance, Magneton was considered an OU Pokémon in Generation II solely because it was used as a counter for Skarmory, another commonly used Pokémon. In Generation IV Magnezone, the evolution of Magneton, took over its place in OU, due to increased stats and a broader movepool. Blaziken was moved from Borderline to Underused to Uber in Generations III through V, skipping designation in the Overused tier entirely.
The Uber tier is effectively a banlist for the OU tier (described below), as "standard" battles include everything in the OU list and below. Pokémon in the Uber tier are deemed too powerful or destabilizing to the metagame to allow. Ubers, if allowed, would cause a too-substantial "centralization" of strategies to those who create the tiers.
The Uber tier may include any Pokémon, not just legendary Pokémon. Not all legendary Pokémon are Uber, however; most members of the various non-mascot legendary trios are allowed in OU play, and even Black Kyurem is allowed in standard play (despite being restricted in-game) due to various flaws preventing it from becoming too centralizing.
Pokémon in Borderline, commonly referred to as "BL", are considered by some to be too strong for Under-Used, but are not sufficiently used in competitive battles to be deemed "overused." In effect BL serves a ban list for Under-Used.
Pokémon in Borderline 2, or "BL2", as it is more commonly known, are considered to be too powerful for the RU/LU tier but are not competitive in UU. As such, it serves as a banlist of the RU/LU tier.
Under-Used, commonly referred to as "UU", designates Pokémon whose use is limited as they are not competitive in OU competition. Pokémon classified as Under-Used are often outclassed in stats, available moves, abilities, or a combination of those by Pokémon in higher tiers. For example, in Generation V Machamp is designated as "UU" due to Conkeldurr having Mach Punch and better bulk. UU Pokémon can be, and are, used in OU battles, but are usually underpowered here.
This tier is referred to as either "RU" or "LU". While the name of the tier varies, its "level" is equivalent. It serves to designate Pokémon who are not used commonly used in the Under-Used (or higher) tiers. For example, Nidoqueen in Generation V is considered RU because it is rarely used in UU battles.
Never-Used, commonly referred to as "NU", is for Pokémon who are not frequently used in the RU/LU tier. The name is not literal, but reflects the extremely limited utility the Pokémon in the tier. Most novelty Pokémon reside in this tier. Some Pokémon that are pre-evolutions of Pokémon that are in the OU or Uber tier may be classified as NU. Many NU Pokémon are common in Pokémon Contests and Pokémon Super Contests.
Little Cup is where the weakest Pokémon, (frequently lowest evolutions), are allowed. Examples include Pokémon like Bulbasaur and Timburr. Pokémon that do not evolve at all, such as Luvdisc, are generally not allowed in the tier, and some unevolved Pokémon are deemed too powerful for Little Cup and therefore banned. In Generation V, Too-powerful Pokémon that have not yet evolved include Gligar, Carvahna, and Meditite among others. It follows the same rules as the Pokémon Colosseum and Pokémon Communication Club Colosseum special rule.