Fish is an animal category that encompasses many species of water dwellers, and even a few species that do not spend all that much time in the water. There are several basic characteristics that are common to most fish, but only a few, like the presence of gills, can be applied to all fish. In fact fish are the most diverse animal species, among the vertebrates, with 32,000 members according to FishBase, a comprehensive database of information on fish used by researchers and zoologists.
One of the basic criteria for an animal to be classified as a fish is the presence of gills throughout its life cycle. Gills are a necessity for underwater life. Whales, for instance, do not have gills; rather, they have lungs, and this is one of the reasons that they are classified as mammals and not fish. Tadpoles have gills, but eventually lose them in their transformation into frogs.
Another characteristic of many fish, though not all, is the presence of scales. Scales are a common evolutionary adaptation, meaning that they have come about through evolution on several separate occasions. Different kinds of scales exist, like the rough, sometimes sharp scales of a shark and the sharp, pointy scales of a blowfish. Scales shield the body of the fish from the environment, protect the fish from predators, reduce drag for more efficient movement or perform some combination of those characteristics. Some fish, like eels, however, have no scales.
Fins are an almost universal characteristic of fish. Many types of fins exist, but the most common are the tail fin, a matching pair of side fins, dorsal fins, and an anal fin. The general purpose of fins is to provide motion and maneuverability. However, there are no set shapes, sizes, or even specific purposes for a particular kind of fin. Whereas many fish often use the tail fin to provide motion, a seahorse, for instance, uses its dorsal fin instead for the same purpose and has no tail fin.
Another characteristic of fish is that they live in water. This may seem too obvious to be worth stating, but there are some fish that can spend significant amounts of time out of the water. Mudskippers, for instance eat and interact with each other on land, and often go underwater only to hide from predators. They do not have lungs, but can breath through their skin and keep their gills moist with sea water that they store internally.
Almost all fish are cold-blooded, or ectothermic, which means they rely on the outside environment to regulate their body temperature. This is the opposite of mammals, for example, which rely on internal processes to maintain a constant body temperature. Fish and many other cold-blooded animals are sensitive to environmental changes like global warming for this physiological reason.