So Nice to BITE You!
Sharks are a particular type of fish that have a very precise definition that separates them from other fish.
Sharks have a cartilaginous skeleton, 5-7 gill slits on their sides near their heads, and also have a distinctive fin arrangement, including pectoral fins.
Sharks date back more than 420 million years, which is older than most dinosaurs, let alone humans. In fact, sharks were around before there was any life on land and there weren't even very many plants.
The point is that sharks have been around for a long time, though all we know of the earliest sharks is learned from incomplete skeleton pieces.
The great white shark and a few others, though, maintain a higher internal body temperature. Sharks breathe through their gills by taking oxygen out of the water.
It is a misconception that all sharks must keep moving or die - most can stay still, but then they have to pump water over their gills in order to continue breathing. Only a few species of sharks must swim endlessly in order to survive.