Lizards are reptiles. Together with snakes, they make up the order Squamata.

There are about 6,000 species, which live all over the world, except in cold climates. They range across all continents except Antarctica, as well as most oceanic island chains.  One type, the marine iguana, lives in the sea. Size varies greatly, from geckos of a few inches or cm to the Komodo dragon of 3 meters (9 feet) and 70 kg (150 pounds). 
 
Some kinds of lizard are: 
 Basilisk
 Tories 
 Gecko
 Iguana
 Monitor lizard
 Frill-necked Lizard
 Draco, flying lizard
 Slow-worm: a lizard without legs.

Simplified classification 
Suborder Lacertilia (Sauria) 
 Infraorder Iguania: Iguanas, chameleons, agamas and relatives.
 Infraorder Gekkota: Geckos, legless lizards, blind lizards.
 Infraorder Scincomorpha: Skinks, wall lizards and relatives.
 Infraorder Anguimorpha (Platynota, Varanoidea): Monitor lizards, Gila monsters, Slow-worms and relatives.
 † Family Mosasauridae: the dominant large marine predators of the Upper Cretaceous.
 Infraorder Amphisbaenia: legless burrowing worm lizards.

Alternative view 
In the traditional taxonomy the Order Squamata is divided as follows:
Order Squamata (the scaled reptiles)
 Suborder Serpentes (snakes)
 Suborder Lacertilia (lizards)

A modern view is that the snakes and lizards are all infraorders of the Squamata:p238

Order Squamata
 Infraorder Serpentes
 Infraorder Iguania
 Infraorder Gekkota
 Infraorder Scincomorpha 
 Infraorder Anguimorpha (Platynota, Varanoidea)
 Infraorder Amphisbaenia
There are other versions, and the taxonomy will probably not settle until more molecular evidence is collected.

Natural history

Anatomy 
The skull structure of both snakes and lizards is distinctive. They can move their upper jaw relative to the braincase. They bear horny scales, and many use venom for attack and defense.

Evolution 
The Squamates are definitely a monophyletic group; they are a sister group to the Tuatara.  Judged by their fossil record, the Squamates were present in the Mesozoic, but occupied a minor place in the land ecology. Three of the six lines are recorded first in the Upper Jurassic, the others in the Cretaceous. Probably all (including snakes) arose earlier in the Jurassic. The Mosasaurs of the Upper Cretaceous were by far the most successful of all the lizards, becoming the top predator in their ecosystem.

Although snakes and lizards look so different, neither are proper clades. Snakes did descend from early lizards, so both groups together do form a monophyletic clade, the Squamata. Within that clade there is another monophyletic clade, the Toxicofera. This includes all venomous reptile species, as well as many related non-venomous species. The evidence for this is in recent molecular analyses.

Physiology

Sight is very important for most lizards, both for locating prey and for communication. Many lizards have highly acute color vision. Most lizards rely heavily on body language, using specific postures, gestures and movements to define territory, resolve disputes, and entice mates. Some species of lizard also utilize bright colors, such as the iridescent patches on the belly of Sceloporus. These colors would be highly visible to predators, so are often hidden on the underside or between scales and only revealed when necessary.

The dewlap is a brightly colored patch of skin on the throat, usually hidden between scales. When a display is needed, the lizards erect the hyoid bone of their throat, resulting in a large vertical flap of brightly colored skin beneath the head which can be then used for communication.

Images

References 

Reptiles