From Bulbapedia, the community-driven Pokémon encyclopedia.
Power Points (Japanese: パワーポイント Power Points), PP for short, are the energy that a Pokémon requires in order to perform a move.
In the games
Power Points have existed in every generation, being a reason for healing Pokémon at a Pokémon Center. Each move is assigned a base Power Point value that is either 1 or a positive multiple of 5, up to 40. In general, weaker moves learned at lower levels will have higher PP, while more powerful moves that are learned by TM only or at high levels will have lower PP.
When a move is learned, its PP will automatically be set to the base PP value, allowing it to be used immediately. However, with TMs in Pokémon Black and White being reusable, this not the case. The PP value will be the same as it was before, regardless of the maximum PP value of the new move, unless the new move has less base PP than the remaining PP of the replaced move.
When PP of a move has been depleted, the Pokémon will no longer be able to use that move until PP is restored. When all of a Pokémon's moves' PP has been depleted, ordering it to attack will result in the Pokémon using Struggle, causing great harm to itself and minimal damage to its opponent.
All moves that target a Pokémon with Pressure use two PP per use instead of one, causing them to deplete their PP faster. A move that has only 1 PP remaining will execute as normal in these situations. Moves that do not target the Pokémon with Pressure, such as status moves which target the user, deplete as normal.
A move that can be used outside of battle, such as Dig or one of the many HM moves, will be able to be used regardless of its PP on the field, and will not subtract PP for its overworld uses.
Upon using Transform, all copied moves will have 5PP (unless the maximum PP is less than 5, in which case the PP will be full).
Some moves, especially the Shadow moves of Pokémon Colosseum and XD, have no PP value, giving them effectively infinite PP.
All moves, except those which have a base PP of 1, can have their usability increased using a PP Up or PP Max. PP Ups boost the move's PP by 20% of the original value per PP Up, and can be used up to three times on the same move. PP Maxes boost a move's PP to 160% of the original value and can only be used once, counting as 3 PP Ups. If a PP Max is used on a move that has already been boosted by a PP Up, it will boost the PP to whatever the maximum would be, serving as two PP Ups if one has already been used, and as one if two have been.
PP can be restored using Elixirs and Ethers, with the plain Ether restoring 10 PP for one move, Max Ether restoring full PP for one move, plain Elixir restoring 10 PP for all of a Pokémon's moves, and Max Elixir restoring full PP for all of a Pokémon's moves. The PP of all party Pokémon's moves will be restored when they are healed at a Pokémon Center, while the PP of individual Pokémon will be restored if they are deposited into the PC.
PP can also be restored using Leppa Berries, which restores 10 PP for one move, the Berry equivalent of Ether.
A Pokémon switched in after a Pokémon has used Lunar Dance will have its PP fully restored.
Base value alteration
In Generations I and II, the maximum PP of a move that began at 40 PP would be 61, probably due to a lack of data space; this is fixed from Generation III onward, increasing the maximum to its "proper" value of 64.
Between Generation III and Generation IV, the base PP of several moves, such as Recover and Giga Drain, was altered. If PP Ups had been used on one of these moves in Generation III, this is carried over to Generation IV, with the PP Ups now boosting based on the new value, rather than the old. A Pokémon with Giga Drain with 2 PP Ups used on it in Generation III, having 7 PP for that move, would have 14 PP when transferred to Generation IV, for example.
A notable glitch found in Generation I allowed struggling to be avoided by allowing the game to self-select a move to be used, which could happen to any move used immediately after a Pokémon was defrosted, or due to a handful of moves' effects (Bind, Clamp, Fire Spin, Hyper Beam, Metronome, Mimic, and Wrap) because of the auto-selection involved with partial trapping moves. A move used with 0 PP in this way would underflow to the maximum possible value, 63 PP, and, due to the way the data is structured, a move that 0 PP Ups had been used on would gain full PP Up status, while those on which PP Ups had been used would lose one PP Up boost. This glitch was addressed in Generation II games and later, which prevent a move from being executed if it has 0 PP.
In the anime
While, like other stats, PP has not been directly referenced by the anime, many Pokémon have been seen to have trouble using a specific move repeatedly, such as Ash's Pikachu's increasing weakness using Thunderbolt on Mewtwo's Poké Balls as they chased him down. Pokémon also get visibly exhausted over the course of a battle, even if they don't take any hits.