The confusion between the words "venomous" and "poisonous" is one of those things that's so commonplace, I'm surprised when I find someone using the terms correctly. Which is unfortunate, because there are few things as pedantic as telling someone "I think you meant to use the word "venomous" instead, good sir." And yet, the difference between the terms is so obvious that it's almost impossible not to do so.
It's somewhat understandable that people might get the terms mixed up. After all, venom and poison are both harmful toxins that can hurt and even kill you. And unless you're an ER doctor treating a patient, there are few meaningful differences between venom and poison; they are both harmful substances. (Few people mistakenly use the word "venom" when they mean "pollen" or "milk").
I find it interesting that "poisonous" is the more common term. You rarely see someone use "venomous" when they mean "poisonous." It's almost always the other way around. Presumably we learn what the word "poisonous" means relatively early in life, what with Mr. Yuck stickers and warnings about not eating unfamiliar berries. When our minds reach for a word that means "toxic substance," it is "poisonous" which comes first to hand.
This is understandable, but somewhat ironic, given that many varieties of venom are not actually poisonous. In the sense that you can actually drink them without experiencing ill effects. It's only when the substance is injected into the blood stream that it becomes harmful.
The primary difference between venom and poison is that venom is always injected. It is the act of injection that makes it harmful. It may be injected by sharp fangs, as by a cobra or spider. Or it may be injected by a stinger, as from a scorpion or wasp. It can also be injected through tentacles bearing specialized injection cells called "nematocysts," as by a jellyfish or sea anemone.
Poison, however, is toxic when ingested or inhaled. There are not very many animals which are poisonous, with the notable exception of the liver of the fugu pufferfish, and the poison dart frog. Many plants, berries, and mushrooms are poisonous, including deadly nightshade, poison hemlock, and the death cap mushroom.
I'm not aware of any poisonous snakes. You can eat all of them, so far as I know. But there are many venomous snakes, whose bite is painful if not fatal.
When trying to determine if something is venomous or poisonous, ask "Who's biting whom?" If it's deadly when you bite it (as is the case with the death cap mushroom), then it's poisonous. If it's deadly when it bites you (like the rattlesnake or black widow spider), then it's venomous.