There are five major types of living reptiles, including lizards, crocodilians, turtles, birds and tuataras. While many popular references list snakes as a distinct form, they are firmly nested within the lizard clade. Additionally, many people are unaware that birds are a type of reptile.
Tuataras are living fossils that have remained virtually unchanged for millions of years. Known only from New Zealand, tuataras resemble lizards, but they have many subtle anatomical differences. For example, tuataras have well-developed parietal eyes and they have different reproductive structures than lizards do.
Lizards are the largest grouping of reptiles. Including snakes, they are represented by over 9,000 species. Lizards and snakes differ in a number of ways, but for all of the differences there are exceptions. For instance, while snakes lack legs, some lizards lack legs as well. Snakes lack eyelids and ear openings, but so do some lizards. Snakes are thought to have evolved from lizards millions of years ago.
Crocodiles and birds actually share common ancestors, and many scientists suspect that birds are the direct descendants of dinosaurs. While birds have very different biological traits than other reptiles, they indisputably arose from within reptilian evolutionary lines. Turtles are very different from any other animals on the planet, and are the most distantly related of any of the living reptiles. No other living animal has a shell similar to that of a turtle.