Fishes are vertebrates with scales, fins and gills. They are exclusively aquatic. There are three major types of fishes: Jawless fishes (Cyclostomes), Cartilaginous (Chondrichthyes) and Bony (Osteichthyes) fishes. Cyclostomes (Jawless fishes) have a circular aperture called mouth. They lack paired fins and are called lamprey and hag fish. These are found in North America.
Chondrichthyesand vs Osteichthyes (Cartilaginous vs Bony fishes)
Cartilaginous fishes (Chondrichthyes)
Examples:Sharks, rays and Skates 
1. Cartilaginous endoskeleton.
2. Gills are exposed to the outside.
3. Mouth on the ventral side of the head.
4. Swim bladder absent.
5. Placoid scales.
6. Fertilization is internal.
Bony fishes (Osteichthyes) fishes
Examples: Salmon, Flying fish, Sea horse, Rohu, trout
1. Bony endoskeleton.
2. Gills covered by operculum.
3. Mouth at terminal end of the head.
4. Swim bladder present.
5. Cycloid and ctenoid scales.
6. Fertilization is external.
