Take poisonous and venomous as a prime example. Both of them seem to indicate that plant or animal has some kind of poison that is harmful and potentially lethal. That’s true, but there is a critical distinction between these two words.
Something that is poisonous contains some sort of poison that is administered passively. There are certain frogs in the rainforest, for instance, where their skin is coated in a poison. If you don’t touch, eat, or lick these frogs, you’re relatively safe. Similarly, there are fruits and other plants that are poisonous only when digested.
By contrast, something that is venomous contains poison that is administered more actively. A king cobra snake, as an example, is venomous, because it administers its toxin (venom) by biting you. A range of insects are similarly venomous, perhaps administering their toxin by a sting rather than a bite.
Just like how less and fewer have similar meanings, poisonous and venomous are not to be used interchangeably in the context of the plant and animal kingdom. When referring colloquially to a situation or a person, though, the meaning can be almost one and the same.