What are tiers?
Tiers in Pokémon are ranks or classes, and each Pokémon fits into one of these tiers based on how often it is used in the standard competitive tier: OU, or Overused. Pokémon used often competitively are placed in that tier. Pokémon that are used competitively, but not as often, are placed in the lower tiers, such as UU (Underused).
The tiers go as follows from highest to lowest: Anything Goes, Ubers, Overused, Borderline, Underused, Borderline 2, Rarelyused, Borderline 3, Neverused. The Borderline tiers are actually banlists for the tier that follows each and as such do not support actual metagames.
For each tier there is also a banlist. Pokémon in these banlists can only be used in the tiers above those that they are banned from. For example, Pokémon in Borderline, or BL, which is the banlist for Underused (UU), can only be used in OU-up because they are too powerful for the lower tiers. In Anything Goes, however, any Pokémon may be used because it is the final banlist.
Pokémon can change between tiers at times too, either upwards or downwards. These tier shifts are determined largely by metagame trends, developing strategies and playstyles, and so forth. Tier shifting is a process run in the form of three-month checks on each Pokémon's usage percentage in battles in the Overused tier. A special type of tier shift called a suspect test is also run. These suspect tests put select Pokémon on the chopping block for immediate expulsion to higher tiers and banlists, as they are deemed possibly too powerful and/or unhealthy for the tier they reside in.
Even though usage statistics and suspect tests generally dictate a Pokémon's placement, there are some cases in which a Pokémon on a banlist has enough usage to be in the tier above it but is kept in place anyways. This situation occurs when a Pokémon placed in a banlist is treated as being part of the tier but will not officially be listed in it to prevent the need for wasteful future suspect tests should its usage drop enough to put it back in the tier it was banned from. These Pokémon will stay in that banlist except in the case that there is significant usage that causes it to jump above the tier it is treated as being in (e.g. a Pokémon in BL2 being treated as UU gets enough usage in OU to jump into it).
AG
Anything Goes (AG) is a new tier that literally is defined by its name. There are no banned Pokemon and only one clause currently - the Endless Battle clause. This construction allows for a metagame where everything is viable and is as close to a replication of the cartridge environment as there ever was. This tier was created as a result of the controversy surrounding the dominance of Mega Rayquaza in Ubers; things were taken into the hands of Smogon's owner and turned out this way. It is still a developing metagame, though it will likely never be as serious as the Ubers tier of the past that it is supposed to be a spiritual successor for.
Ubers
Ubers is a banlist-turned-tier with the invention of AG, and all Pokémon may be used in it as opposed to the other banlists (with the exception of Mega Rayquaza), which are quite small and not nearly as popular as the actual tiers. As mentioned previously, the Pokémon making up this tier were much too powerful for use in the OU tier. They warped team building around themselves, forcing teams to carry counters which many of these Pokemon don’t have, or use the overpowered Pokémon themselves.
OU
Overused is the standard tier and is home to the Pokémon used most often competitively. They usually have better stats, abilities, movepools, and typing than the Pokémon in lower tiers, which are the main reasons they are used more often. In OU, every Pokémon is allowed except for those that are banished to the Uber tier. However, this does not mean that every Pokémon is viable.
Lower tier Pokémon are unviable in the OU tier for a number of reasons. Sometimes, there is a Pokémon in OU that does the same job better. The Pokémon’s stats could also hold it back, meaning that some Pokémon are simply not powerful enough for standard use. Nonetheless, there is still the chance that some lower tier Pokémon are perfectly viable in OU, and you should not be discouraged from trying them.
BL
Borderline is a list of Pokémon that are not used often enough to be OU, but are too game-changing and powerful to be in UU. This means that they can only be used in the OU and Uber tiers.
UU
Underused is the tier directly below OU. The Pokémon here are not used as often as those in OU, as indicated by the name of the tier, and are oftentimes not as powerful. All Pokémon in this tier can also be used in the OU and Uber tiers.
BL2
Borderline 2 is a banlist for the Rarelyused tier. These Pokémon, similarly to BL, are too powerful to be in the RU tier, but are not used often enough to be UU. They can be used in UU-up.
RU
Rarelyused is a relatively new tier, created during the fifth generation to help accommodate with the influx of new Pokémon and also because the Neverused tier was very large. It has helped make it so that almost every Pokémon can be a valuable part of a team in at least one tier. All Pokémon in NU may also be used in this tier, and all Pokémon in this tier can be used in every tier above it.
BL3
This is the banlist for the Neverused tier. Like those placed in other Borderline tiers, the Pokémon here are too powerful for use in NU but are not used often enough to be placed in RU. These can be used in RU and up.
NU
The Neverused tier is home to the Pokémon that are barely played in standard, if at all. These Pokémon generally have lackluster stats, a bad movepool, a crippling ability, or a combination of the aforementioned. Only NU Pokémon may be used in this tier, but, because it is still large, there is still room for varying team compositions. The Pokémon in this tier can be used in all other tiers as well.
Tiers in Pokémon are ranks or classes, and each Pokémon fits into one of these tiers based on how often it is used in the standard competitive tier: OU, or Overused. Pokémon used often competitively are placed in that tier. Pokémon that are used competitively, but not as often, are placed in the lower tiers, such as UU (Underused).
The tiers go as follows from highest to lowest: Anything Goes, Ubers, Overused, Borderline, Underused, Borderline 2, Rarelyused, Borderline 3, Neverused. The Borderline tiers are actually banlists for the tier that follows each and as such do not support actual metagames.
For each tier there is also a banlist. Pokémon in these banlists can only be used in the tiers above those that they are banned from. For example, Pokémon in Borderline, or BL, which is the banlist for Underused (UU), can only be used in OU-up because they are too powerful for the lower tiers. In Anything Goes, however, any Pokémon may be used because it is the final banlist.
Pokémon can change between tiers at times too, either upwards or downwards. These tier shifts are determined largely by metagame trends, developing strategies and playstyles, and so forth. Tier shifting is a process run in the form of three-month checks on each Pokémon's usage percentage in battles in the Overused tier. A special type of tier shift called a suspect test is also run. These suspect tests put select Pokémon on the chopping block for immediate expulsion to higher tiers and banlists, as they are deemed possibly too powerful and/or unhealthy for the tier they reside in.
Even though usage statistics and suspect tests generally dictate a Pokémon's placement, there are some cases in which a Pokémon on a banlist has enough usage to be in the tier above it but is kept in place anyways. This situation occurs when a Pokémon placed in a banlist is treated as being part of the tier but will not officially be listed in it to prevent the need for wasteful future suspect tests should its usage drop enough to put it back in the tier it was banned from. These Pokémon will stay in that banlist except in the case that there is significant usage that causes it to jump above the tier it is treated as being in (e.g. a Pokémon in BL2 being treated as UU gets enough usage in OU to jump into it).
AG
Anything Goes (AG) is a new tier that literally is defined by its name. There are no banned Pokemon and only one clause currently - the Endless Battle clause. This construction allows for a metagame where everything is viable and is as close to a replication of the cartridge environment as there ever was. This tier was created as a result of the controversy surrounding the dominance of Mega Rayquaza in Ubers; things were taken into the hands of Smogon's owner and turned out this way. It is still a developing metagame, though it will likely never be as serious as the Ubers tier of the past that it is supposed to be a spiritual successor for.
Ubers
Ubers is a banlist-turned-tier with the invention of AG, and all Pokémon may be used in it as opposed to the other banlists (with the exception of Mega Rayquaza), which are quite small and not nearly as popular as the actual tiers. As mentioned previously, the Pokémon making up this tier were much too powerful for use in the OU tier. They warped team building around themselves, forcing teams to carry counters which many of these Pokemon don’t have, or use the overpowered Pokémon themselves.
OU
Overused is the standard tier and is home to the Pokémon used most often competitively. They usually have better stats, abilities, movepools, and typing than the Pokémon in lower tiers, which are the main reasons they are used more often. In OU, every Pokémon is allowed except for those that are banished to the Uber tier. However, this does not mean that every Pokémon is viable.
Lower tier Pokémon are unviable in the OU tier for a number of reasons. Sometimes, there is a Pokémon in OU that does the same job better. The Pokémon’s stats could also hold it back, meaning that some Pokémon are simply not powerful enough for standard use. Nonetheless, there is still the chance that some lower tier Pokémon are perfectly viable in OU, and you should not be discouraged from trying them.
BL
Borderline is a list of Pokémon that are not used often enough to be OU, but are too game-changing and powerful to be in UU. This means that they can only be used in the OU and Uber tiers.
UU
Underused is the tier directly below OU. The Pokémon here are not used as often as those in OU, as indicated by the name of the tier, and are oftentimes not as powerful. All Pokémon in this tier can also be used in the OU and Uber tiers.
BL2
Borderline 2 is a banlist for the Rarelyused tier. These Pokémon, similarly to BL, are too powerful to be in the RU tier, but are not used often enough to be UU. They can be used in UU-up.
RU
Rarelyused is a relatively new tier, created during the fifth generation to help accommodate with the influx of new Pokémon and also because the Neverused tier was very large. It has helped make it so that almost every Pokémon can be a valuable part of a team in at least one tier. All Pokémon in NU may also be used in this tier, and all Pokémon in this tier can be used in every tier above it.
BL3
This is the banlist for the Neverused tier. Like those placed in other Borderline tiers, the Pokémon here are too powerful for use in NU but are not used often enough to be placed in RU. These can be used in RU and up.
NU
The Neverused tier is home to the Pokémon that are barely played in standard, if at all. These Pokémon generally have lackluster stats, a bad movepool, a crippling ability, or a combination of the aforementioned. Only NU Pokémon may be used in this tier, but, because it is still large, there is still room for varying team compositions. The Pokémon in this tier can be used in all other tiers as well.