Rain forests are forests characterized by high rainfall, with definitions setting minimum normal annual rainfall between 1750–2000 mm (68-78 inches). The monsoon trough, alternately known as the inter-tropical convergence zone, plays a significant role in creating Earth's tropical rain forests.

A total of 40 to 75% of all species on the world's habitats are indigenous to the rain-forests. It has been estimated that many millions of species of plants, insects, and microorganisms are still undiscovered. Tropical rain forests have been called the "jewels of the Earth", and the "world's largest pharmacy", because over one quarter of natural medicines have been discovered there. Rain forests are also responsible for 28% of the world's oxygen turn over, often misunderstood as oxygen production,processing it through photosynthesis from carbon dioxide and storing it as carbon through bio sequestration.

The undergrowth in a rain forest is restricted in many areas by the lack of sunlight at ground level. This makes it possible to walk through the forest. If the leaf canopy is destroyed or thinned, the ground beneath is soon colonized by a dense, tangled growth of vines, shrubs, and small trees called a jungle. There are two types of rain forest, tropical rain forest and temperate rain forest.