This page describes many of the features about fish and explains why they are related and how they differ. Fish are comprised of two main groups, the Elasmobranchs or cartilaginous fish and the Bony fish. The shark uses its oily liver for buoyancy and glides on its fins.
Some sharks have to always swim to maintain buoyancy.
The powerful heterocercal tail drives muscular thrust through the shark's centre of gravity assisting with buoyancy. Fish have the swim bladder, a buoyancy control device. It is another reason why fish are more advanced. Interesting, isn't it. Scales are overlapping plates made of the same material as bone.