Lizards are reptiles so they share many common reptile traits.They are cold-blooded (poikilothermic). They have scaly skin. Most have four pentadactyl limbs. Most lay hard shelled eggs. There are exceptions to every rule of course and there are both legless lizards and lizards that bear live young. Unlike mammals, the legs of lizards are not placed under the body but jut out from the sides. Normally a lizard's tail is longer than its body but there are exceptions to this too.
There are four orders of living reptiles: turtles and crocodiles make up two orders. The tuatara of New Zealand is the only living representative of its order. It looks very lizard-like on the surface but has some important primitive characteristics that separate it. All the rest are snakes and lizards and belong to the order Squamata. If it has legs, it is a lizard. If it doesn't have legs then things get more difficult as there are two groups of legless lizards. If the legless animal has external ear openings, then it is a lizard. It is also a lizard if the length from the vent to the end of the tail is longer than the length from the snout to the vent. However both these clues involve getting up close and personal with the animal, which given the number of poisonous snakes in the world, may not be a good idea. When in doubt, leave it alone!
There are about twenty families of lizards. The largest family, with nearly four thousand species, is the Agamidae. These lizards occur throughout Africa, Eurasia and Australia.They are quite diverse but usually have stout bodies and long tails. They can be arboreal, terrestrial or semi-aquatic. The bearded dragon is an example of an agamid. The New World equivalent family are the iguanas (Iquanidae) with 700 species in the Americas. Another two of the most numerous are the geckos, with six hundred species (Gekkonidae) and the skinks with another 600 species (Scincidae). Geckos have relatively large heads while skinks have narrow rounded bodies and small pointed heads. Geckos are unique among lizards because they can vocalise. If it barks, it's a gecko. Some skinks are legless and others have reduced legs.
There are also many small families with weird and wonderful forms of lizards. The basilisks, chameleons, alligator lizards, Gila monsters, limbless skinks and burrowing slow 'worms' are all types of lizards, but classified in their own families because they are so unique. The Gila monster and its close relative, the beaded lizard, of the American southwest, are the only poisonous lizards in the world. Monitors are among the most impressive lizards (Varanidae). The 40 odd species are found around Australia and southeast Asia. The largest lizard in the world, the Komodo dragon, is a varanid, as are the goannas of Australia.
In general, lizards are predators of insects and other invertebrates. Most lizards will also eat some plant material. My bearded dragon is particularly fond of dandelion flowers. Larger lizards can catch larger prey and many will also eat carrion. I was told a story by an old bushie once. He came across a dead cow during a drought and all of a sudden it looked like its tongue was moving. When he got closer, it turned out to be the tail of a goanna which had crawled inside the dead cow and was eating it from the inside out! Goannas are not poisonous but because of their carrion eating habits, their mouths are filthy and bites can easily become infected.
Lizards may not be warm blooded or bear live young but they are not primitive and they are great survivors. After all, they survived the catastrophe that wiped out the dinosaurs. Many lizards make good pets and there is information on the Internet if you are interested in getting one. Many lizards are now threatened by human activities and need to be protected. They are important contributors to the diversity of life on earth and the healthy functioning of the earth's ecosystems.
http://animal-world.com/encyclo/reptiles/information/lizardclassfamilies.php is a good site for information on the various lizard families. I also used information from Cogger 2000: Reptiles and Amphibians of Australia