From Bulbapedia, the community-driven Pokémon encyclopedia.
An elemental type is a property for Pokémon and their moves. Each elemental type has three properties: which types of Pokémon it is effective against, which types of Pokémon it is less effective against, and which types of Pokémon it is completely ineffective against. There are 17 types, each of which is listed to the right.
A Pokémon may have either one or two types. For instance, Charmander is a Template:Type2, while Bulbasaur is both a Template:Type2 and a Template:Type2. With the current 17-type system, there are a possible 272 different type combinations. A move may have only one type.
Within the anime and the games, most Gym Leaders and members of the Elite Four are designed to have a type-specific theme.
If the type of a move used by a Pokémon is strong against the opponent's type, then the damage done is twice the normal; if a move's type is weak against a specific type, then the damage done is half the normal damage done. Note that this effect is multiplied, e.g if a move's type is strong against both of the opponent's types (such as Dig, a Ground-type move on an Aggron, a Steel/Rock Pokémon), then the move does 4 times the damage.
If a move's type is completely ineffective due to the opposing Pokémon's type, then the damage done by the move is zero. Otherwise, the damage will be at the very least 1HP.
Type-affected game mechanics
All games before Generation IV also made a distinction between physical and special moves based on their type; as of Generation IV, this is dependent on the move itself.
When the type of an attack corresponds with one of the types of the Pokémon using it, the attack power will be increased by 50%. This type advantage is referred to as Same-type attack bonus, or STAB for short. A good example is an Aron that knows Metal Claw and Cut. Both attacks are physical attacks and both have a base power of 50. However because Aron is a Steel/Rock Pokémon, the power of its Metal Claw move will be multiplied by 1.5.
Some weather conditions, abilities, and held items affect moves of a certain type. Sunny Day, for example, causes Template:Type2 moves to increase in power, while Levitate causes Template:Type2 moves to not work on the Pokémon with this ability. Likewise, each type has a specific item that can be given to a Pokémon to hold that will power up one of the specific types, such as the Metal Coat, which powers up Template:Type2 moves by 20% (10% prior to Generation IV).
There are several glitch types, such as the Bird type, which only appear through the use of glitches. Other than the Bird type, glitch types are the result of the game reading other data as if it were types. The Bird type was programmed into the code of the Generation I and II games, but was not given to any Pokémon.