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Pokemon Go: what are Pokemon Types for?
Pokemon in Pokemon Go all fall into one of a few categories types. A Pokemon can have one or two types, and types determine a number of things about that Pokemon. For one it determines where the Pokemon likes to live (more on that under the next heading), but it also determines battle strengths and weaknesses.

Theres 18 different types, and each type is either strong, weak or neutral to other types. At the most basic level with the starter Pokemon this breaks down to Rock, Paper, Scissors Charmander (Fire) beats Bulbasaur (Grass) who beats Squirtle (Water) who in turn beats Charmander.

The way types and battles in general work in Pokemon Go is marginally different to the major Pokemon games, but the same core concepts apply. In Pokemon Go each Pokemon has two moves. As well as the Pokemon itself having one or two types, its moves will also have types. A shortcoming in a Pokemons natural typing may be helped by the types of its moves.

Fire-type Pokemon Strengths and Weaknesses
Effective Against: Steel, Bug, Ice, Grass
Weak Against: Rock, Water, Ground

Water-type Strengths and Weaknesses
Effective Against: Fire, Ground, Rock
Weak Against: Electric, Grass

Grass-type Pokemon Strengths and Weaknesses
Effective Against: Water, Ground Rock
Weak Against: Fire, Ice, Poison, Flying, Bug

Electric-type Pokemon Strengths and Weaknesses
Effective Against: Water, Flying
Weak Against: Ground

Normal-type Pokemon Strengths and Weaknesses
Effective Against: None
Weak Against: Fighting

Ice-type Pokemon Strengths and Weaknesses
Effective Against: Grass, Ground, Flying, Dragon
Weak Against: Fire, Fighting, Rock, Steel

Poison-type Pokemon Strengths and Weaknesses
Effective Against: Grass, Fairy
Weak Against: Ground, Psychic

Fighting-type Strengths and Weaknesses
Effective Against: Normal, Ice, Rock, Dark, Steel
Weak Against: Flying, Psychic, Fairy

Ground-type Pokemon Strengths and Weaknesses
Effective Against: Fire, Electric, Poison, Rock, Steel
Weak Against: Water, Grass, Ice

Rock-type Pokemon Strengths and Weaknesses
Effective Against: Fire, Ice, Flying, Bug
Weak Against: Water, Grass, Fighting, Ground, Steel

Psychic-type Pokemon Strengths and Weaknesses
Effective Against: Fighting, Poison
Weak Against: Bug, Ghost, Dark

Bug-type Pokemon Strengths and Weaknesses
Effective Against: Grass, Psychic, Dark
Weak Against: Fire, Flying, Rock

Flying-type Pokemon Strengths and Weaknesses
Effective Against: Grass, Fighting, Bug
Weak Against: Electric, Ice, Rock

Dragon-type Pokemon Strengths and Weaknesses
Effective Against: Dragon
Weak Against: Ice, Dragon, Fairy

Fairy-type Pokemon Strengths and Weaknesses
Effective Against: Fighting, Dragon, Dark
Weak Against: Poison, Steel

Ghost-type Pokemon Strengths and Weaknesses
Effective Against: Psychic, Ghost
Weak Against: Ghost, Dark

Dark-type Pokemon Strengths and Weaknesses
Effective Against: Psychic, Ghost
Weak Against: Fighting, Bug, Fairy