Turtles are reptiles classified into the taxonomic order testudines. They are characterized by the shells that cover much of their bodies. Turtles can be found on all continents except for Antarctica, and tend to inhabit areas with temperate and tropical climates. Some species of turtle are specialized to live in water, while others reside mostly on land. There are about 300 extant species of turtle.

A turtle's shell is comprised of an upper shell and a lower shell, which are connected by a bony structure at the turtle's side. Each shell consists of multiple segments that fit together to form the respective shell. It is generally made up of an inner layer of bone, which is connected to the turtle's internal bone structures; this layer of bone is most often covered by a horn material made of the protein keratin. A turtle's shell serves to protect it from any potential predators by providing a sturdy protective structure into which it can withdraw all exposed body parts when it feels threatened.

In addition to their shells, turtles are characterized by four limbs, a lack of teeth, tails, and necks that either fold or contract back into their shells. Land turtles, most accurately called tortoises, have bent legs and short and hefty feet. Amphibious turtles, or those that can live on both land and in water, tend to have similarly structured legs and feet, only their toes are webbed and have long claws. Entirely aquatic turtles have flippers instead of legs.

Turtles can be found in a wide range of sizes. The smallest living turtle is a South African species called the speckled padloper tortoise, which is a maximum of about 3 inches in length and an average weight of about 5 ounces. The largest extant turtle is the critically endangered leatherback sea turtle, which is a maximum of about 6.5 feet in length and may weigh over 2,000 lb.

The diet of turtles depends heavily on their habitat. Tortoises in drier regions tend to be herbivores, and are not known to be selective in choosing their food. Tortoises living near or in water are more often omnivorous, and may feed on aquatic plant life, insects, and sometimes dead aquatic life. Sea turtles are primarily carnivorous, and may eat marine organisms with soft bodies or shellfish; some species, however, are entierly herbivorous, and eat plant life such as algae.

Turtles reproduce by laying eggs that are either spherical or elliptical in shape. Mother turtles leave large numbers of eggs in holes in mud or sand, and generally leave the site in which they laid their eggs soon after; mother turtles do not care for their offspring in their youth. Eggs then incubate for an average of two to four months. After hatching, some sea turtles tend to seek water immediately, while tortoises may remain in an embryonic sac until they are strong enough to move and find food in their environments. Turtles may go on to live for 20-150 years, depending on species.