The origin of the term Amphibia comes from Latin meaning 'two lives' (land and water based), as most amphibians spend a larval stage as aquatic water breathers.
A Beaver is a vertebrate and a mammal.
they are both the same things they are an animal with a backbone
The difference is that invertebrates don't have a backbone. Vertebrates have a backbone.
Fishes are vertabrates because they have bones remember, this causes fossils.