Turtles are reptiles with hard shells that protect them from predators. They are among the oldest and most primitive groups of reptiles, having evolved millions of years ago. Turtles live all over the world in almost every type of climate.

According to the Integrated Taxonomic Information System (ITIS), the turtle order, Testudines (or Chelonia), splits into two suborders, Cryptodira and Pleurodira, and then further splits into 13 families, 75 genera and more than 300 species.

"Turtle," "tortoise" and "terrapin" are often used interchangeably as synonyms, but there are distinct differences between the types of chelonians, according to the San Diego Zoo:



Turtles spend most of their lives in water. They are adapted for aquatic life, with webbed feet or flippers and a streamlined body. Sea turtles rarely leave the ocean, except to lay eggs in the sand. Freshwater turtles live in ponds and lakes, and they climb out of the water onto logs or rocks to bask in the warm sun.
Tortoises are land animals. Their feet are round and stumpy, adapted for walking on land. They also dig burrows with their strong forelimbs, and slip underground when the sun gets too hot.
Terrapins live on land and in water, usually in swamps, ponds, lakes and rivers.
Size

With so many different types of turtle, there is no average size. The largest sea turtle species is the leatherback turtle.