Fish are aquatic vertebrates. They usually have gills, paired fins, a long body covered with scales, and tend to be cold-blooded. Fish come in different colours and shapes. There are many kinds of fish: the small, the big, the black, and the coloured.

There are actually three main classes, groups, or types of fish: bony fish, jawless fish, and cartilaginous fish. Fish are the most diverse group among the vertebrates, with over 33,000 different types of fish species. However, not all aquatic creatures which resemble fish are considered fish. Whales, dolphins, and porpoises are aquatic mammals, for example.

All of the fish take oxygen from the water they swim in. They take it with their gills. Fish scientists are called ichthyologists. They have discovered many types of strange fish and they are expected to discover new species of fish in the coming years.