A venom is a particular kind of poison. Mathematically speaking, venoms are a proper subset of poisons. (Meaning the all venoms are poisons, but not all poisons are venoms.)
Venoms are usually injected.
venoms are produced by animals. Poisons are not
Venon is always injected. It is not usually fatal unless it gets to blood stream. So at least in theory, you could swallow snake venom and it would be broken down in your stomach just like other foods.
If you eat something and you get sick or die, it's poisonous. If something bites you and you get sick or die, it's venomous.