The appearance and size of the shell varies greatly with the species. Turtle species range in size from about 11 cm to 2 meters. Aquatic species usually have webbed feet and use their lungs as air or swim bladders, causing air to flow into different parts of the lung. They are capable of learning and have significant memories. Their external sexual characteristics are difficult to distinguish.
Turtles appear worldwide on land and in oceans with temperate to warm environments.
All turtles are hatched from eggs, and all turtle eggs are laid on land. The shape of eggs and number in each batch varies with species. They are slow growers, and their shells display rings showing uneven growth periods, much like the rings of a tree trunk.