Venomous vs. poisonous
There is a difference between organisms that are venomous and those that are poisonous, two commonly confused terms applied to plant and animal life. Venomous, as stated above, refers to animals that deliver (often, inject) venom into their prey when hunting or as a defense mechanism. Poisonous, on the other hand, describes plants or animals that are harmful when consumed or touched. A poison tends to be distributed over a large part of the body of the organism producing it, while venom is typically produced in organs specialized for the purpose. One species of bird, the hooded pitohui, although not venomous, is poisonous, secreting a neurotoxin onto its skin and feathers. The slow loris, a primate, blurs the boundary between poisonous and venomous. From patches on the inside of its elbows it secretes a toxin, which it is believed to smear on its young to prevent them from being eaten; however, it will also lick these patches, giving it a venomous bite
Snakes are not poisonous and frogs are not venomous. Venom is a toxic substance that is injected. Certain species of snakes, scorpions and spiders are venomous, not poisonous. Venom glands typically form the toxic substance and the venom is stored until it is needed. The venomous animal will then bite or sting another creature, whether as intended prey or in defense, and the venom will be injected. Depending on the amount injected, the susceptibility of the animal injected and the size of the animal, various degrees of illness, including death, can occur.
If you call a snake poisonous, you are actually implying that the snake has a toxic substance on his body and poisoning will occur if the snake is handled. This does not occur. Venom is used primarily to immobilize prey and is rarely used as a defense mechanism. The venomous animal will bite in an attack but injecting venom is usually reserved for prey items.
A poison is a substance that is absorbed through the skin or ingested, resulting in toxicity. Certain amphibians, fish and insects secrete a substance that is poisonous. The poisonous animal does not inject the substance into another creature. The substance is either absorbed through the skin or ingested when the poisonous animal is placed in the mouth or swallowed. Poison is typically used as a defense mechanism and is rarely used to incapacitate prey.