From Bulbapedia, the community-driven Pokémon encyclopedia.
An analysis of which types can be hit for Super-Effective Damage. For example, a Fire type move will do super-effective damage against Bug, Grass, Ice, and Steel types. As Super-Effective moves are the most efficient way to take out other Pokémon, trainers usually strive for good type coverage on individual Pokémon, and for the whole team. Hidden Power helps as a wild-card to allow a Pokémon to cover types it wouldn't normally be able to, such as a Charizard with Hidden Power Water, which would then give it coverage against other Fire types. Ice is typically a good type to have, as the most commonly seen Dragon types have a double weakness to the Ice type.