The Poison type is one of the eighteen official types from the Pokémon franchise. The Poison type is one of the more defensive types in the franchise, resisting five types and being weak to just two. A bunch of Poison moves make the target poisoned or badly poisoned, quite irritating status ailments to have. Poison's two weaknesses, Ground and Psychic, are unfortunately quite common in competitive battling, but those aren't that much of a threat when the Poison type is also Bug type, which is a common type Combination for Poison Pokémon.
Offensively, it won't go very well. It is resisted by four types, and won't have any effect on Steel Pokémon. For a long time, Poison moves were only super effective against the Grass type, and despite that type having six other weaknesses, Trainers are better off with Fire or Ice type Pokémon, as there is a large portion of Grass/Poison type Pokémon (Poison being resisted by itself). A bunch of the heavier Grass-type Pokémon can retaliate with a powerful Earthquake.
As of Generation 6, however, Fairy became an additional type, which became weak to Poison. Still, however, Poison should still be used with caution, as some of the Fairy Pokémon are also Psychic type, and can retaliate with moves of that type. It won't have any effect on Mawile and Klefki at all due to their Steel typing. But there are a good bunch of Fairy type Pokémon that the Poison type still can be useful against.
As of Generation 6, there are 59 official Poison type Pokémon in existence, making it more of an uncommon type, but still quite common. This amount is also tied with the Fire type. There aren't actually many pure Poison-type Pokémon, and when it comes to dual-typed Poison Pokémon, Poison is more popular as a secondary type. Most of the dual-typed Pokémon of this type are Bug or Grass type as well.
Poison has a mixed bag of subjects, but most appear to take resemblance to venomous animals that are able to poison other organisms in real life. In fact, the Bug/Poison type Pokémon always seem to be based around poisonous or dangerous insects and other arthropods. There are also some Pokémon based around toxic matter such as gas, sludge or garbage. There's also a Poison type Pokémon based on a stomach.
Grass/Poison Pokémon do not appear to be based on venomous animals, but rather poisonous or smelly plants. Amoongus, despite resembling a mushroom instead of a plant, is an example of the poisonous plant, while Vileplume is an example of a smelly plant.