Status conditions
Burn, poison, paralysis, sleep, freeze, confusion and friends, with the exact numbers used in the newest games.
A status condition is a problem that follows a Pokémon: damage over time, lost turns or weaker stats. Each kind has a type that is immune to it, a berry that cures it, and some moves that cause it. The numbers below are the current ones (Generation IX), with older values noted where they differ.
Two kinds
Non-volatile conditions (burn, poison, bad poison, paralysis, sleep, freeze) stay when a Pokémon switches out and after the battle ends, until cured. A Pokémon can only have one at a time. Volatile conditions (such as confusion, infatuation or flinching) wear off when the Pokémon leaves the battle, and a Pokémon can have several at once.
Burn
A burned Pokémon loses 1/16 of its maximum HP at the end of every turn (it was 1/8 from Generation II to Generation VI). Burn also halves the damage from its physical moves, unless it has the Ability Guts or uses Facade. Fire types cannot be burned. Water Veil prevents it, and Rawst Berries cure it.
Poison and bad poison
Poison takes 1/8 of the maximum HP every turn. Bad poison (from Toxic) starts at 1/16 and rises by another 1/16 each turn that the Pokémon stays in, so it hurts more the longer it lasts, but the counter resets if the Pokémon switches out. Poison and Steel types are normally immune, unless the attacker has the Ability Corrosion. Poison Heal turns the damage into healing.
Paralysis
A paralysed Pokémon has a 25% chance of being unable to move on any turn, and its Speed is halved (it was cut to a quarter before Generation VII). Electric types cannot be paralysed in recent games. Limber blocks it, and Cheri Berries cure it.
Sleep
A sleeping Pokémon cannot use moves, except Snore and Sleep Talk. In recent games sleep lasts 2 to 4 turns. If a Pokémon puts itself to sleep with Rest, it is fully healed and sleeps for 2 turns after the turn it uses Rest (3 turns counting that one). Insomnia and Vital Spirit prevent sleep.
Freeze
A frozen Pokémon cannot move, but each turn it has a 20% chance to thaw. Being hit by a Fire move melts it at once, and a few Fire moves such as Flare Blitz thaw their user. Ice types cannot be frozen, and in harsh sunlight nobody can be frozen at all. Legends: Arceus replaced freeze with frostbite, a burn-like condition that takes 1/12 of maximum HP and halves special damage.
Confusion and other volatile conditions
A confused Pokémon lasts 2 to 5 turns. Each turn it has a one-in-three chance (33%) of hurting itself instead of acting, with the same damage as a typeless physical move of power 40 and no critical hits. Switching out cures it.
Other volatile conditions include infatuation from Attract (a 50% chance of not moving), flinching (the Pokémon misses the rest of the turn), Leech Seed (1/8 of HP drained every turn), Taunt, Encore and Torment, and Yawn, which sends a Pokémon to sleep at the end of the next turn.
Protecting against status
Safeguard, Misty Terrain (for grounded Pokémon), a Substitute and the Abilities Leaf Guard (in sun) and Comatose all prevent status conditions. Lum Berries cure any condition once. Legends: Arceus and Legends: Z-A replace sleep with drowsiness. In Legends: Arceus a drowsy Pokémon may fail to act and takes extra damage, and in Legends: Z-A it doubles the cooldown on its moves.