A typical fish is torpedo-shaped, with a head containing a brain and sensory organs, a trunk with a muscular wall surrounding a cavity containing the internal organs, and a muscular post-anal tail. Most fish propel themselves through the water by weaving movements of their bodies and control their direction by means of the fins. All have skins covered with slimy glandular secretions that decrease friction with the water; in addition, nearly all have scales, which together with the secretions form a nearly waterproof coating. All fishes have a lateral line system of sensory organs for detecting pressure changes in the water. All have water-breathing organs called gills located in passages leading from the throat, or pharynx, to the exterior; a few fishes also have air-breathing lungs as an additional means of respiration. In all but the most primitive class, the gill passages are supported by skeletal structures called gill arches. Plankton-feeding fish have structures called gill rakers attached to the gill arches; these strain minute organisms from the water as it passes out of the pharynx. Fish breathe by taking water into the mouth and forcing it out through the gill passages; as the water passes over the thin-walled gills, dissolved oxygen diffuses into the gill capillaries and carbon dioxide diffuses out. The circulatory system is closed, and the heart is two-chambered; the blood is red. With few exceptions, fish are cold-blooded; that is, they cannot regulate their body temperature, which is the same as that of the environment.

Methods of reproduction are varied. Sharks have internal fertilization, and most give birth to live young. Those that lay eggs produce large ones with tough shells. Since embryonic development is well-protected in these fish, they produce a relatively small number of young, only seven or eight at a time in some species. A few of the bony fishes, including some aquarium species, are live bearers, but most lay small, unprotected eggs that are fertilized after deposition in water. In most marine species the eggs float freely in the currents, where they are eaten by other animals. An enormous number of eggs is therefore necessary to ensure the maturation of a few; in many species a female produces as many as 5 million eggs in one spawn. The eggs of most marine fishes contain oil droplets that buoy them up, while those of most freshwater fishes are heavy, with sticky surfaces that adhere to objects in the water. Most freshwater species build nests for the protection of the eggs, and in some the adults guard the nests.