Fish are aquatic vertebrates that breathe through gills, have fins, lay eggs, and have scales covering their skin. There are two main types of fish: cartilaginous fish like sharks and rays whose skeletons are made of cartilage; and bony fish like salmon and carp whose skeletons are made of bone. Bony fish make up the largest group and can live in both saltwater and freshwater environments. All fish are aquatic vertebrates that use gills to breathe and fins to swim.