A rainforest is a dense, damp forest with a huge number of different kinds of plants and animals. Scientists believe there may be millions of species in rainforests that haven't been discovered yet.
There are two kinds of rainforests, tropical and temperate. Temperate rainforests are much smaller, and there are fewer of them. Most rainforests in the United States (in the Pacific Northwest and Alaska) are temperate. Tropical rainforests are remarkably wet and warm, with frequent rain and thick growth of trees and plants. Hawaii has the US's only tropical rainforests.