Both venomous and poisonous organisms can be harmful, sometimes deadly, but the difference between them lies in the delivery of their toxins. A venomous organism injects its venom into other organisms using either a stinger or fangs, which are attached to a gland that produces the venom.
The venom is injected often to immobilise prey or protect against potential predators. It is therefore usually always intentional. Poisonous organisms are different in the sense that large parts of them, or often the entire organism, contain toxins. Therefore eating, or even just touching it, can prove harmful.