Hemlock is poisonous, but rattlesnakes are venomous. Both terms refer to toxic chemicals, so what's the difference between poison and venom?
The difference is entirely in how the species uses its defense mechanism. If it's injected—via a snake's bite or a bee's sting or the spiney protrusions on a frog's head—it's a venom. If it's secreted through the skin, as in the case of poison dart frogs or a few species of birds, it's a poison. Poisons work their deadly magic through touch, ingestion, or inhalation, while venoms are injected directly into a wound.
If you have to choose between being bit by a venomous snake or touching a poisonous frog, you might want to go with the snake. As it turns out, snakes don't always inject venom when they bite: Some 20 percent or more of bites from venomous snakes are "dry bites" that create a wound but don't include venom. Poisonous animals can't choose when to apply their toxins.
For more, check out this explanation from TED-Ed:
This article originally ran in 2015.