Bilateria refers to a major group of animals characterized by bilateral symmetry, meaning their bodies can be divided into two mirror-image halves. This group includes a wide variety of organisms, from insects and fish to mammals. Bilaterians typically have a distinct head and tail, as well as a dorsal (top) and ventral (bottom) side. Bilateria is divided into two main clades: Protostomia and Deuterostomia. Protostomes, like arthropods and mollusks, develop their mouth first during embryonic development, while deuterostomes, including chordates and echinoderms, develop their anus first. This classification helps scientists understand evolutionary relationships among different animal groups.