Competitive battling is quite advanced in the Pokémon games. This article covers the strategies used in the metagame as opposed to casual players. This requires an understanding of stat mechanics, moves and Pokémon with the right attributes. Smogon is a website detailing different competitive strategies. Pokémon are grouped into tiers to determine their usage.
Movesets in general are much more specified than ones used in casual play. Rather than using four attacking moves of different types, movesets are instead suited to a Pokémon's different skills. Boosting moves used to increase Attack or Special Attack such as Swords Dance and Nasty Plot are quite popular within the metagame, but moves boosting defensive stats, such as Amnesia or Acid Armor, are never used due to "passivity" within the metagame. Attacking moves with high base power such as Hyper Beam or Thunder are generally unviable because of their large drawbacks, such as a recharge turn or low accuracy. Changing game mechanics mean that some popular moves, such as Explosion or Blizzard may end up abandoned due to their disadvantages.
In competitive battling, natures that decrease an attacking stat are most commonly used, because only one offensive stat needs to be used in a moveset. Adamant, Bold, Impish, Modest, Calm, Careful, Timid, and Jolly are the most commonly used natures. Natures that do nothing (Bashful, Docile, Hardy, Quirky, Serious) are never used in competitive battling.
The Speed stat is generally considered the most important stat in modern battling. Because of this, Pokémon that have naturally higher Speed, such as Infernape and Weavile, are used over slower Pokémon such as Rampardos or Rhyperior. While Trick Room can be used to give slower Pokémon an advantage, faster Pokémon get to attack first, and Speed tiers are formed to determine the percentage of Pokémon that can be outsped. Offensive stats are the second most important stats, and defensive stats (HP, Defense and Special Defense) are used for walls or tanks.
EVs and IVs
EVs are distributed differently in many different Pokémon. Starting in Generation III, EVs can max out only two stats and add a small amount to a third stat. For example, Gengar would be given 4 Defense EVs, 252 Special Attack EVs, and 252 Speed EVs, because Speed and Special Attack are its best stats. In the first two Generations, all of a Pokémon's stats could be maxed out, so now maximum bulk was impossible.
Introduced in Generation III, abilities changed the competitive scene. Abilities such as Intimidate and Wonder Guard can change the defensive usage of a Pokémon. The addition of weather-related abilities to several Pokémon in Generation V meant that several Pokémon that were previously useless, such as Politoed and Ninetails. The large amount of playstyles in Generation V meant that entire teams around weather could be built.
Types are an important part of competitive battling. Statistically, the Dragon type is the strongest, with generally high attacking stats, but only hits itself for super-effective damage. The Ground and Fighting types are the most super-effective types, and so can be combined with a second type for coverage, such as Rock/Fighting or Ground/Ice. Defensively, the Steel and Fairy types are popular due to the former's large amount of resistances and the latter's resistances to popular types such as Dragon and Fighting. The Ghost type is also prominent for its immunity to two types.
Holdable items introduced in Generation II have become a staple of modern battling. Items such as Assault Vest, Leftovers and Black Sludge are used to regenerate health or increase defensive stats for certain Pokémon, while Choice Scarf, Choice Specs and Choice Band are used on offensive Pokémon. The introduction of Mega Evolution meant that previously unviable Pokémon, such as Charizard or Mawile now had a role in the metagame. Mega Evolutions typically have high base stats on par with Legendary Pokémon, and also feature important abilities.
Changing game mechanics have a huge impact on competitive battling. The split of the Special stat into Special Attack and Special Defense had a massive impact on many Pokémon; Gyarados, Exeggutor and Tauros suffered due to getting one of their stats lowered, while Jynx and Snorlax benefited due to increased stats. The physical/special split was one of the biggest changes to competitive Pokémon, as previously some Pokémon had no way to take advantage of their STAB moves. For example, Gengar's Attack stat is poor, and both of it's STAB types are physical. The physical/special split introduced special Ghost and Poison type moves. In Generation I, Speed determined critical hits, so Pokémon like Persian and Mewtwo were overpowered.
Moves that boost evasion (i.e. Double Team and Minimize) are banned.
At least two Pokémon on a team cannot be frozen at the same time.
At least two Pokémon on a team cannot be asleep at the same time. Self-induced sleep such as Rest does not activate Sleep Clause.
One-Hit KO moves are not allowed, such as Guillotine.
Two or more of the same Pokémon can not be used on the same team, such as a team of different forms of Arceus.
Self KO Clause
If both players have only one Pokémon left, moves which KO both the user and the opponent are not allowed, such as Explosion or Destiny Bond. If recoil damage would cause a draw, Self KO Clause does not take place, and the player who attacked last is the winner.
All Pokémon on a team must hold different items. This is not a normal clause in competitive play, but it is used in Nintendo tournaments.