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 top and bottom, which allows for more complex movement and organization. Bilateria is divided into two main subgroups: Protostomes and Deuterostomes. Protostomes develop their mouth first during embryonic development, while Deuterostomes develop their anus first. This distinction is important for understanding the evolutionary relationships among different animal species and their developmental processes.