Amniotes are a group of vertebrates that include reptiles, birds, and mammals. They are characterized by having an amniotic egg, which allows them to reproduce on land without the need for water. This adaptation helps protect the developing embryo and provides it with nutrients. The amniotic egg contains several membranes, including the amnion, which surrounds the embryo, and the chorion, which helps with gas exchange. This evolutionary development enables amniotes to thrive in diverse terrestrial environments, making them a significant part of the animal kingdom, alongside other groups like amphibians and fish.