Rain Forest

Rain forests were a lush biome, which existed in the game prior to the Adventure Update. They can be viewed as a forerunner to the Jungle biome seen in later version of Minecraft, due to their denser-than-average vegetation, higher tree concentration and more common large trees.

Description
Rainforests are very wet biomes with many trees, which have a 1 in 3 chance of being large trees, instead of 1 in 10 like all other biomes. They only generate oak trees and have a large amount of tall grass and ferns. A biome is classified as a rainforest if the temperature is greater then 97% and the rainfall is more than 90%. As a result of this, the rainforest can often contain a lot of cliffs and hills because the world generator reduces height variation at lower rainfall values and vice versa.