What are the classical adaptations of amphibians? My question is about amphibians in humid or underwater environments. What biological adaptations can found in them, further what purposes do they serve?

"Amphibian" means "living both in water and on land" (see here). Here is a small list of classic adaptations in amphibians:

* Eyes and nostril are positioned to poke out of the water while the rest of the body is immersed
* Ears don't let any water gets in
* Skin is very thin and allow direct gazeous and water exchange
* Paws are webbed to move into water
* Larvae of most amphibian species start their life in the water