Lizards are a large group of animals belonging to the reptile class (Class Reptilia). Their closest living relatives are the snakes, plus an obscure group of reptiles, the amphisbaenians or "worm-lizards". Together, lizards, snakes and amphisbaenians form the order Squamata. This in turn is related to the other reptilian groups, living and extinct: the crocodiles, turtles and tuatara (an endangered lizard-like creature from New Zealand) of our present age, plus the dinosaurs and other prehistoric reptiles.
Before we go any further I'd like to emphasise that lizards are not dinosaurs and are only indirectly related. The ancestors of lizards, or Eosuchians, appeared about 180 million years ago and existed alongside the dinosaurs, but there are many obvious differences. Most strikingly, dinosaurs have legs tucked straight beneath the body, whereas lizards (or at least those with legs) walk with a sprawling gait. Also, there is much reason to believe that at least some dinosaurs were "warm-blooded" or endothermic, whereas all living reptiles are cold-blooded. Nevertheless, many lizards do have a somewhat dinosaur-like appearance, which is one reason for their appeal today. But don't expect your pet to grow into a Triceratops or Velociraptor.
Reptiles and mammals both belong to the vertebrates, or back-boned animals, a group that also comprises the fish, amphibians and birds. Compared with the other 95% of the animal kingdom, vertebrates share a vast number of common characteristics. However, there are also important differences between reptiles and mammals, which if overlooked can make a captive reptile's life a miserable one.
Lizards can normally be characterised by a dry, watertight and scaly skin, a tail and usually four limbs (although some have none or just two). The limbs are normally held out at a distance from the body, giving the lizards their characteristic side-to-side motion, or "skitter". Apart from these basic traits there is huge diversity amongst lizards, as any trip to a reptile house should demonstrate. Lizards vary hugely in size, shape, colour, habitat and diet, and in their suitability as pets. Some lizards make eminently suitable captives; others should only be kept in zoos, if at all. Some of the better-known lizards are actually less suitable for domesticity, notably the iguana and the chameleon. They can be kept, but with exacting attention.