The main difference between reptiles and critters is the type of animal being referred to. Reptiles are cold-blooded animals such as snakes, turtles or lizards. The word "critters," on the other hand, is a slang term often used to refer to any type of animal, regardless of species. The term is sometimes used to refer to a person as well, usually in a derogatory manner.
There are many types of reptiles throughout the animal kingdom, from small snakes to larger crocodiles and alligators. In fact, many dinosaurs were part of the reptile family. Reptiles also have distinct characteristics that set them apart from other animals, including bodies covered with scales or plates and skeletal differences.
Reptiles are air-breathing creatures that usually have short legs. Some, such as snakes, have no legs at all and move around by slithering on their bellies. Since they are cold blooded, most species are found in warmer climates. Those that do live in cooler regions hibernate during the winter months.
While the word critters is believed to have originated in the early 19th Century, little is known about how or why it became a common term. The word "reptile," however, was first used in the 14th Century and is a derivative of the Middle English "reptil."