Rainforests are forests characterized by high rainfall, with annual rainfall in the case of tropical rainforests between 250 and 450 centimeters. There may be many millions of species of plants, insects and microorganisms still undiscovered in tropical rainforests. 

They 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. Rainforests are also responsible for 28% of the world's oxygen turnover, sometimes misnamed oxygen production, processing it through photosynthesis from carbon dioxide and consuming it through respiration. More than half of the world's species of plants and animals are found in the rainforest.

One of my great passions would be to make great journeys through the rainforests, however, I know all the difficulties involved so you can only hike in some rainforests like Borneo, Philippines, Indonesia, and in some areas of Africa.

Sources:  Olson, David M.; Dinerstein, Eric; Wikramanayake, Eric D.; Burgess, Neil D.; Powell, George V. N.; Underwood, Emma C.; d'Amico, Jennifer A.; Itoua, Illanga; et al. (2001). "Terrestrial Ecoregions of the World: A New Map of Life on Earth" (PDF). BioScience. 51 (11): 933–938.