From Bulbapedia, the community-driven Pokémon encyclopedia.
Types play as large a part in the Pokémon TCG as they do in the video games. Both Pokémon and Energy cards can be one or more of these types. Similarly, a Pokémon can and probably will have a Weakness and Resistance to other types.
Unlike in the games, there are only eleven known types. Prior to the Fairy type being introduced, there were only ten known types.
Also, unlike the games, when a Pokémon attacks, the type of damage it does is based on the Pokémon's own type and not the type of Energy the attack requires. For example, although Dark Golduck uses Psychic energy to power both of its attacks, the attacks do double damage against a Pokémon weak to Water because Dark Golduck's TCG type is Water in this instance.
Because of the limited number of types, many of the 18 types found in the games have been combined into one of the TCG types. For example, Fighting-, Rock- and Ground-types in the games are instead combined into the Fighting in the TCG. However, in keeping with Pokémon tradition, the Weakness and Resistance of certain Pokémon are similar to their weaknesses in the games. For example, even though Glalie is a Water-type in the TCG, it is weak to Metal, much like how in the main series, Glalie, as an Ice-type, is weak to Steel.
Another difference between the games and the TCG is how damage is calculated according to Weakness and Resistance. Beginning with Base Set, if a Pokémon had a weakness to another Pokémon's type, that damage done was doubled, and if a Pokémon had a resistance to another Pokémon's type, that damage was reduced by 30 damage points. This system was set and did not vary between Pokémon, types and sets. However, beginning with the Diamond & Pearl set, the Weakness and Resistance system was changed slightly to allow for variation for how damage was adjusted, such as a Weakness that added 30 damage instead of doubling it, or a Resistance reducing damage by 20 instead of 30.
Sometimes, special Pokémon, such as Pokémon-ex and the like, will have more than one Weakness and/or Resistance. Likewise, other special Pokémon may have more than one type, a feature introduced in the EX Team Magma vs Team Aqua expansion, with Pokémon that were part Darkness and part their normal type. Eventually, the EX Holon subseries would introduce Pokémon with another secondary type, this time Metal.
Despite this standard categorization, however, some Pokémon may be featured as a different type.