Fish are very diverse animals and can be categorised in many ways. Although most fish species have probably been discovered and described, about 250 new ones are still discovered every year. Fish are water animals that live in freshwater or saltwater and breathe through gills. Some fish are small and colorful, while others are large and used for food, fishing, or aquarium keeping. Salmon, tuna, goldfish, trout, shark, and catfish are some common examples of different types of fish. Fishes may be seen to be distributed according to habitat and geographical area. All aspects of the life of a fish are closely correlated with adaptation to the total environment, physical, chemical, and biological. Instead, there are 3 main classes, groups, or types of fish: bony fish (Osteichthyes), jawless fish (Agnatha), and cartilaginous fish (Chondrichthyes). Fish are the most diverse group among vertebrates, with over 33,000 different types of fish species. Fish have been an important natural resource for humans since prehistoric times, especially as food. Commercial and subsistence fishers harvest fish in wild fisheries or farm them in ponds or breeding cages in the ocean. All fish share two traits: they live in water and they have a backbone—they are vertebrates. Apart from these similarities, however, many of the species in this group differ markedly from one. Fish are vertebrates (vertebrates have backbones) that live in water. They breathe using special organs called gills. Fish (plural: fish or fishes) are a group of animals which live in water and respire (get oxygen) from their gills. As a group, they are much older than other vertebrates. fish, any of approximately 34,000 species of vertebrate animals (phylum Chordata) found in the fresh and salt waters of the world. Living species range from the primitive jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes. Fish are extraordinarily diverse aquatic animals, and a useful overview has to go well beyond the familiar image of a scaly, perch-shaped swimmer. Living fishes include jawless hagfishes and lampreys, cartilaginous sharks and rays, the enormous radiation of ray-finned fishes, and living lobe-finned fishes such as lungfishes and coelacanths. They occupy fresh water, salt water, brackish.