Most fish are streamlined in form. Streamlining produces the least resistance to movement through the water. The shape of the fish generally depends on its way of life. Fish living on the ocean floor are usually flat bodied. Slow-swimming fish have a rounded body, often protected by spines or armor plates. The body of the fish is supported by a skeleton, consisting primarily of a skull and a backbone. Most fish have teeth on the jaws or on bones further down the throat. Scales cover the body of most fish. There are some, however, such as the catfish, that have no scales and are covered with a thick skin. Fish "breathe" by ventilating their gills with a current of water. The gills are richly supplied with blood and are able to extract oxygen from the water.

Almost all fish show a clear distinction between sexes. Males may be distinguished from females by the form and size of the fins, the length of the barbels (whisker-like process on the lip) or the position of the eyes. The majority of fish lay eggs which are fertilized outside the body of the fish.