What is the difference between dolphins, fish and sharks?

Dolphins are mammals.  This means that they breathe air, give birth to live young and nurse those young.  Fish however, lay eggs and breathe underwater through their gills.  Sharks are a bit different. They are fish, but some give birth to live young.  However, all sharks also breathe through their gills.  Sharks' skeletons are made out of cartilage, not bones.