Venom, acid, smog, spores, toxic sludge... some individuals have suggested that the Poison type be renamed the Chemical Warfare type, as it would be more accurate. However, common nomenclature has already cemented their identity, so these pokemon are stuck with the name of poison even in cases where this would not be entirely accurate. Almost every poisonous pokemon has bright purple or blue markings on their body as a warning to potential threats; the few that don't are immature, predatory, or social in nature.
It should be noted that some out of date pokedexs record poison types and bug types to be mutually lethal to each other. This misunderstanding arises from the fact that, while poison attacks affect bug types normally, the bug's smaller body usually succumbs quicker then larger organisms; in retaliation, many social bug types preemptively swarm poison types, leading researchers to find their corpses and reach the wrong conclusion. Most modern pokedexs have clarified this error.
The various acids, venoms, and spores of the poison type allow them to affect their opponents in a myriad of ways ranging form inducing hallucinogenic visions to temporary blinding, from melting off their skin to poisoning their blood, from paralyzing agents to artificially induced seizures. Their near instinctive understanding of biochemistry only furthers this power, as they are resistant to each other's poisons. Fighting type pokemon pull their punches to avoid these effects, and while bug type pokemon hate poison types they find their attacks are somewhat ineffective against the double layer of skin poison types tend to have to contain their venoms. Grass types are especially vulnerable to poison types, as their plant-like cardiovascular systems increase the flow of venom and their usual methods of assault are partially neutralized by the poison type's internal poison.
However, the poison type's biochemical mastery comes at the price of over-specialization. Their reliance on their venoms to induce status effects makes pure poison types rather unused to pure aggression, and rock, ground, and ghost types can protect themselves from venoms to some degree, while Steel types use their armor to prevent ever being injected in the first place. The ludicrously powered Ground type assaults typically have more impact on the poison type, who are not designed to fight the environment. Psychic types also take advantage of the poison type's specialization, fiddling with the mental processes that control their venoms and opening safety valves, which cause poison types to fall prey to their own powers.
Because poison type pokemon are overspecialized, the vast majority of poison types are in fact hybrids; ironically, their foes in bug type and grass type pokemon are the ones that most often take poison type as their secondary group. The relatively rare pure types tend to have fangs, be covered in quills, or actually be amorphous forms containing their powerful acids and spores.
Due to widespread hybridization, their foods vary widely, but the more amorphous species can and do consume anything up to and including toxic substances.
Varies widely, but amorphous species reproduce through fission.