Typically, Reptiles and certain water-borne animals are armed with Venom. Smaller land insects and animals are generally armed with Poison. Consider the case of snakes and scorpions or even your little red ant or a honey bee. They all have fangs or stingers as a weapon. A weapon which is used both for defense and hunt. They generally have a hypodermic needle-like setup. On the bite, they deliver a deadly or at times a very irritating mix of chemicals. These chemicals or venom depending upon its source can either kill you or give you a lot of pain.
Venom is generally secreted by a specialized gland and that gland is always in proximity to a fang or stinger.
Poison, on the other hand, is spread over a large surface area of the animal. Typically the skin has poison glands. The poison in the animals make them non-palatable and keeps predators at bay. Some poisons are local irritants and are less deadly. For example, certain types of frogs have a toxin which burns the skin on mere touch.
Venom may range from non-lethal to extremely lethal depending upon the animal. The quantity of the venom also decides its nature. For example, a simple bee sting will make you cry, but on the other hand, a snake bite if unattended might kill you.
The type of venom or poison that a creature carries is also different it might be a neurotoxin, a blood coagulating type or even a blood hemorrhaging type.
All in all the holy-grail of differentiation is that a venomous animal will inject the toxin, whereas a poisonous animal has to be ingested or come in contact with for toxin to work.