For the most part, the terms "turtle" and "tortoise" are used to refer to the differences in where the animals live, and their adaptations to these environments, but the terms do not indicate a strict taxonomic separation. "Turtle" is often used for both turtles and tortoises.

The earliest known turtles date from 215 million years ago, making turtles one of the oldest reptile groups, even more ancient than lizards and snakes. About 300 species are alive today, although some are highly endangered.

Tortoises have life spans comparable with those of humans, some are known to have lived longer than 150 years.

Like other reptiles, turtles are cold-blooded, their body temperature depends directly on the temperature of the air or water around them, they breathe air, and lay eggs on land, although many species live in or around water.

Although many turtles spend most of their lives underwater, all turtles and tortoises breathe air, and must surface at regular intervals to refill their lungs. Many also spend much of their lives on dry land.

Land and aquatic turtles come in a wide range of sizes, colors, and shapes.

The turtle's protection comes from a shell is composed of hard, bone plates covered by scutes. The scutes are made of keratin, the primary substance in hair, nails and hooves of other animals. The pigment melanin, present in the scutes, may form intricate designs and brightly colored patterns in some species.

Turtles lay eggs, like other reptiles, which are slightly soft and leathery. Large numbers of eggs are deposited in holes dug into mud or sand, then they are covered and left to incubate by themselves. When the turtles hatch, they squirm their way to the surface and head toward the water.