Statistics (Stats) are values that apply to a Pokémon's abilities in battle. These values determine such factors as, damage given, damage received, which Pokémon acts first, etc.. Most statistics are affected by the individual Pokémon and their level, while others are exclusive to battle and moves. Effort Values determine the amount that stats grow when leveling up.
HP or Health Points is a value that determines how much damage a Pokémon can receive. When a Pokémon's HP is completely down to 0, the Pokémon will faint. HP is the most frequently affected stat of them all, as depleting HP is the key factor in winning a battle. There are several ways to replenish a Pokémon's HP such as using items like Potions or in-battle moves like Rest or Synthesis.
HP is depicted as a long, horizontal bar that changes color according to how much HP the Pokémon has left along with numerical values showing the current and total HP. When a Pokémon's HP is full or above 50%, the bar will appear green. Below 50% will cause the bar to appear yellow, while critical conditions will cause it to appear red.
Attack is a value that determines how much damage a Pokémon will cause to the opponent while using a physical move. Certain items and moves may increase a Pokémon's Attack stat, generally temporarily, though an item such as Protein can increase it permanently.
Defense determines how much damage a Pokémon will resist when hit by a physical move. Certain items and move can increase a Pokémon's Defense stat, generally temporarily, but an item such as Iron can increase it permanently.
Special Attack is a value that determines how much damage a Pokémon can cause while using a special move. In Generation I games, the title for this stat was simply named "special." There were types that signified if the attack was physical or special. Not only did this stat represent Special Attack, it also represented Special Defense. The vitamin Calcium raises both the "Special" stat and the Special Attack stat.
By the Generation II games, the "Special" category separated and created a more accurate special attack and special defense stat. Another change appeared for special attack when Pokémon Diamond and Pearl was released. The system now refers attacks that are special to involve some kind of energy that allows the Pokémon to hit the opponent without making physical contact.
Special Defense determines how much damage a Pokémon will resist when hit by a special move. As explained in the Special Attack section, during the Generation I games the stat known as "Special" incorporated both Special Defense and Special Attack. With the release of the Generation II games, "Special" was divided into two stats. The vitamin Calcium permanently raised Special Defense during the Generation II games but was changed to Zinc, which is currently used now.
Speed is a value that determines which Pokémon will act first during battle. Generally, the Pokémon with the higher Speed will be the one to attack first. If one Pokémon uses a move such as Quick Attack, Speed values are no longer a factor. If both Pokémon use such a move, the factor that determines who will act first is reverted to the Speed stat. If two or more Pokémon's Speed stat are the same, one of the Pokémon will act first in a random order each turn. Carbos will permanently raise this stat.
Accuracy is a percentage value that determines the chance of a Pokémon's attack hitting the opponent. Accuracy values, unlike the previous stats, are not based on an individual Pokémon and can not be increased permanently by leveling up or by vitamins. During battle, the accuracy stat for each Pokémon is at 100% and can be decreased by moves such as Sand-Attack. While the Accuracy stat affects all of a Pokémon's moves, it is not the same as an individual move's Accuracy Rating.
Evasion is a percentage value that determines the chance of an opposing Pokémon's move missing. The Evasion stat for each Pokémon at the start of a battle is at 0%. If the stat is decreased below 0% with a move such as Sweet Scent, then the opposing Pokémons' move will have a larger chance of hitting. If the stat is increased above 0% with a move such as Double Team, the opposing Pokémon's move will have a smaller chance of hitting.