The largest climate in Indonesia is overwhelmingly tropical wet. In the mountains the climate is highlands, and in the southern islands it's tropical wet and dry. All across the islands, however, it is hot and humid, being only slightly lessened in the highlands, due to its high elevation. Indonesia is right on the equator, and the direct sunlight it receives accounts for the tropical climates. Monsoon winds from Asia and the Pacific bring much of the rain in the tropical wet climate, and dry monsoon winds from Australia go over the southern islands, bringing their dry season.
There are very many ecosystems in Indonesia, being the second most biologically diverse country in the world, after Brazil. The most common and broad are tropical rainforest and tropical grassland in the southern islands, however there are several ranges of sea and costal ecosystems, such as beaches, sand dunes, and coral reefs, and regions on swamps and coastal vegetation, due to the fact that it is an island chain and there is a lot of coast. Much of the diversity also comes from the mixture of Asian, Australian and native species which inhabit the islands. Located directly in between the two, the animals migrated in the middle.
Citations: Textbook pages 639-641, CIA Factbook, “Indonesia’s Natural Wealth”, “The Ecology of the Indonesian Seas: Part One”, State of the World 1997: A Worldwatch Institute Report on Progress Toward a Sustainable Society (14th edition).