True tropical plants are native to the hot and humid regions just North or South of the Equator, within either the Tropic of Cancer (North) or the Tropic of Capricorn (South).
These areas include: South and Central America, most of Africa, India, Indochina (lying East of India, and South of China), the Caribbean and South Pacific.
Not only are these regions the hottest and most humid in the world, they are also the regions that contain by far the most amount of plants, as the all year round climate allows for rapid, successful growth. Consequently, most of these types of plants cannot withstand cold temperatures, but temperate gardeners are get around this by using greenhouses, or by growing more weather-tolerant, sub-tropical plants (plants from areas that border the tropics).
Thailand is within the tropical zone, and this is the reason why Thailand is such an exciting area for those interested in tropical plant life.