The tropical rainforests are located (surprise!) in the tropics, in other words, the band north and south of the Equator , in areas where the climate is warm/hot and moist almost all year round. The biggest ones are in the Amazon region, but there are tropical rainforests in Africa and Asia as well.
As to why they're important, I mentioned some reasons in my response to your question above.
As good place to look for more information is the homesite of the Rainforest Foundation:
They have branches and activities in many other countries.