Fish belong to the class Actinopterygii for the majority of modern fish, which are also known as ray-finned fish. Additionally, there are other classes like Chondrichthyes, which includes sharks and rays, and Sarcopterygii, which includes lobe-finned fish. When we refer to fish broadly, we typically think of those that fall into Actinopterygii.
Fish are primarily aquatic animals that breathe through gills and have fins for swimming. They represent a diverse group of vertebrates, adapted to various marine and freshwater environments, showing remarkable variations in size, shape, and habitat.