Reptiles are defined as animals in the class Reptilia, comprised of turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians (tuatara) and includes some 11,000 plus species. Some classification systems consider birds a separate class to reptiles, however, crocodilians are more closely related to birds than they are to other living reptiles. And as such modern cladistic classification systems include birds within Reptilia.
Today all modern non-bird reptiles inhabit all the continents except Antarctica. Reptiles are tetrapod vertebrates, having four limbs, or if you consider snakes, are descended from four-limbed ancestors. Reptiles unlike amphibians do not have an aquatic larval stage, with most reptiles laying eggs. Several species are however viviparous, where the fetus develops within the mother instead of contained in an eggshell and feed their fetuses through various forms of placenta analogous to those of mammals.
Today extant reptiles, range in size from the tiny gecko, Sphaerodactylus ariasae, which can grow up to 17 mm to the huge saltwater crocodile, Crocodylus porosus), which can reach 6 m or more.
Source: Wikipedia
(Turtles and Tortoises)