Tea is certainly one of the most popular drinks in the world, a fierce rival to coffee, and actually more popular in many regions. The drink is derived from the cured leaf of the Tea plant (Camellia sinensis), a member of the family Theaceae.
Diagram of Camellia sinensis.
It is an evergreen plant that grows in tropical or subtropical regions. You can see in the map below the main countries of production. Obviously, China and India dominate much of the market, though I am partial to some of what I have had from Sri Lanka (Ceylon).
Tea Production by Country -- the darker the green, the greater the production.
The different kinds of tea are derived from the different manners of processing, rather than different species. This chart gives you an idea of where we get Green, Black, Oolong, White, and Yellow Teas from:
Chart of Tea Processing Steps for different kinds of tea -- note that Black Tea is red, and Green Tea is green!
To see this extremely useful chart in higher resolution, follow this link: http://upload.wikimedia.org/wikipedia/commons/1/13/Teaprocessing.svg
Of course, many teas involve blends, such as Earl Grey Tea, which is a black tea that also has the oil of bergamot, derived from the rind of the Bergamot Orange (Citrus bergamia) added to it.
This video gives you an idea of the production of tea, as seen in West Bengal and Assam, India:
Time for a cup of tea...