Status condition
Status ailments affect a Pokémon's ability to battle. There are three kinds of status. The first are non-volatile, the second are volatile, and the third lasts while a Pokémon is in battle.
Non-volatile status
Moves which are non-volatile will remain until a Pokémon is healed at a Pokémon Center, or a specific curative item is used, or some status have the ability to wear off on their own. A Pokémon inflicted with a non-volatile status will still be affected after being pulled out of battle (unless they have the Natural Cure ability), and after a battle is over. A Pokémon can only be inflicted with one of these status at a time. In Generation III and beyond, certain abilities will prevent these statuses from taking place, while others will cause them.
Burn
Burn reduces a Pokémon's physical attack by half. Additionally, at the end of a turn, the Pokémon loses 1/8 its maximum hit points (in Generation I, the Pokémon loses 1/16th of its maximum hit points). Fire-type Pokémon, and Pokémon with the Water Veil Ability cannot be burned by Fire-type moves, but the former can be burned by Tri Attack. All moves which can cause burn are Fire-type except Tri Attack.
Freeze
A frozen Pokémon is unable to attack with most moves. Most Fire-type moves used on a frozen Pokémon will remove the frozen status. As of Generation II, freeze has a random, ~10% chance to be cured on its own on the frozen Pokémon's turn. Contrary to popular belief, Sunny Day weather does not cause a quicker thawing. Ice-type Pokémon cannot be frozen by Ice-type moves except in Generation I; however, they can be frozen by Tri Attack (again, except in Generation I). A frozen Pokémon can still use the moves Flame Wheel and Sacred Fire; when used while frozen, these moves will thaw the user, thaw the opponent if possible, and deal damage to the opponent. All moves which cause freezing are Ice-type except Tri Attack.
Paralysis
A Pokémon inflicted with paralysis will be unable to attack ("fully paralyzed") a quarter of the time. Additionally, its speed is reduced to 25% of its previous value. Many moves that cause paralysis are of the Electric type.
Poison
A poisoned Pokémon loses 1/8 of its maximum hit points every turn (in Generation I, it loses 1/16th). Poison-type Pokémon cannot be poisoned. Steel-type Pokémon cannot be poisoned in Generation III (in Generation II, Poison-type attacks can't affect Steel-types, but can otherwise be poisoned, by Twineedle). A poisoned Pokémon also loses 1 hit point for every four steps taken while not in battle. In Generation IV, a Pokémon whose HP is reduced to 1 via poison outside of battle will have the poison status removed. All moves that can poison are of the Poison-type except Twineedle and Secret Power.
Badly poisoned
The status ailment caused by Toxic and Poison Fang is the same as Poison except its damage begins at 1/16 and grows an additional 1/16 every turn, taking 2/16 max hit points the second turn, then 3/16 the third turn, and 4/16 the fourth, and so on. In Generation I and Generation II, switching a Pokémon out of active battle would change badly poisoned to normal poison. As for Generation III, the "badly poisoned" effect will remain even after switching a Pokémon out of battle and back in. After a battle is over, the "badly poisoned" status will become a normal poison. All moves which can badly poison are of the Poison-type.
Sleep
A Pokémon that is asleep is unable to use any attacks (in a situation almost identical to the Freeze condition), except for two special moves which may be used while asleep (Snore and Sleep Talk). Sleep lasts for a randomly chosen duration of 1 to 7 turns (1 to 3 in Stadium). Sleep may be self-induced for 3 turns (inclusive of the initial turn) using the move Rest. No moves that cause sleep cause damage as well.
Volatile status
A volatile status will wear off when a Pokémon is taken out of battle or a battle is over. Many of these status will also wear off after a number of turns pass. All of these conditions may be passed to another Pokémon by using Baton Pass unless stated otherwise.
Attract
An attracted Pokémon cannot attack 50% of the time. A Pokémon can only successfully use Attract on opponents of the opposite gender; unknown-gender Pokémon are both immune to this condition and unable to inflict it on others. Pokémon with the Oblivious ability are also immune to this. Attract status cannot be passed with Baton Pass. Attraction will end as soon as the Pokémon that used it leaves the battle.
Confusion
A Pokémon will hurt itself in its confusion 50% of the time. Confusion wears off after 1–4 attacking turns. This means that turns recharging, such as after using Hyper Beam, and turns unable to attack, such as from paralysis, will not lower the remaining number of turns of confusion. However, a sleeping Pokémon may hurt itself in confusion if using a move such as Snore or Sleep Talk. Multi-turn attacks such as Fly and Dive require that confusion be checked both turns, further reducing the chance of successful attack. Pokémon with the Own Tempo ability are also immune to moves that cause confusion.
Curse
If a Ghost-type Pokémon uses Curse, then the opponent loses 1/4 its maximum hit points every turn.
Flinch
Flinch prevents a Pokémon from attacking. A Pokémon can only be flinched if the opponent attacks first. Pokémon with the Inner Focus ability are also immune to this. Most moves that cause flinching are physical attacks.
This is known as cringing in Pokémon Mystery Dungeon.
Foresight/Odor Sleuth
The opponent's evasion modification will not affect the accuracy of a Pokémon that uses Foresight or Odor Sleuth. In addition, a Normal- or Fighting-type move used by such a Pokémon will affect Ghost-type Pokémon.
Leech Seed
Each turn, a Pokémon hit by Leech Seed loses 1/8th (1/16th in Generation I) of its maximum hit points. The opponent is healed by the same amount. Grass-type Pokémon are immune to Leech Seed.
Mind Reader/Lock-On
When a Pokémon uses Mind Reader or Lock-On, the next damage-dealing move will hit the opponent without fail.
Nightmare
Nightmare only affects a sleeping Pokémon. The sleeping Pokémon loses 1/4th of its maximum hit points every turn. If the sleeping Pokémon awakens, then the nightmare will no longer be in effect. If Baton Pass switched out a non-sleeping Pokémon, then the nightmare will no longer be in effect.
Perish Song
After three turns, all Pokémon who heard the Perish Song will faint. Note that this excludes Pokémon with the Soundproof ability.
Non-volatile battle status
Defense Curl
Using Defense Curl causes the power of Rollout to double for the Pokémon.
Focus Energy
When a Pokémon uses Focus Energy, its critical hit rate increases.
Mean Look/Spider Web
A Pokémon trapped by Mean Look or Spider Web cannot switch until the Pokémon that used the move is defeated or switches. If a trapped Pokémon uses Baton Pass, the Pokémon brought out will still be trapped.
Minimize
A Pokémon having used Minimize will take double damage if hit by the move Stomp (though this behavior is not present in Generation I).
Mist
Mist prevents a Pokémon's stats from being lowered by the opponent's attacks. This Pokémon may still lower its own stats with moves having such an effect.
Substitute
The Pokémon that uses Substitute loses 1/4th of its total HP (rounded down) to make a Substitute which will absorb hits until it "breaks" (damage the Substitute has taken is greater than the HP used to make it). From Generation II onward, Substitutes block all status ailments and will immediately break if a OHKO attack connects with them. In Generation I, however, a substitute only blocks primary status ailments; attacks like Thunder Wave and Spore completely circumvent the Substitute.