Iguanas are large lizards, adapted to drier conditions and with a thick, scaly skin. They make good pets and are yummy (I have not actually eaten one, but I'm told they're delicious). They live throughout South America and up into the temperate areas of North America, where they are America's largest lizard. They also live on any number of tropical islands, leading to a high degree of speciation. The primary way to tell if your large scaly lizard is an iguana of some sort is looking for the presence of pleurodont teeth, that is, the teeth are attached by their sides to the inner rim of the jaw, as opposed to just being rooted straight down into the gums.
Some live on the ground, some spend most of their time in the water, some live in the trees. Some aren't all that large. Some are humongous. Like I said, there's a lot of speciation. If you want more details, select a specific species of iguana from the list below.
Class Reptilia
Order Squamata
Suborder Iguania*
Family Iguanidae:
* Includes anoles and chameleons, among others.