(Note: this was in response to a question asked here: http://chowhound.chow.com/topics/8153... -- The Chowhound Team)
OK, remedial tea 101.
I'm no tea expert, so I'm providing the absolute basics. Other more knowledgeable tea hounds can supplement or add as they see fit.
Anything called "tea" comes from one specific plant. "Herbal teas" like chamomile are not truly "tea", but the word tea has become a shorthand for "any leaf brewed in hot water." True tea comes from the tea plant.
The really short version of this process is that when these green leaves are simply dried, they become green tea (surprise!).
When these leaves are fully fermented and then dried, they become black tea. Think about it... when you ferment something, the bacteria consume and transform the leaves. It's essentially a controlled rotting of the vegetation, which is why the leaves turn black.
Oolongs are somewhere in between. They are partially fermented, and the process of partially fermenting is a much more complicated one. There is more of an artistry that goes into making oolongs, similar to the way a master vintner has to have the palate. An oolong tea maker needs to have the palate to know when to stop the fermentation, how to roll the leaves, how long to dry/roast them. There's an infinite number of variables. The best oolongs are all produced by hand. The cheaper and lower grade ones are produced by factories.
If you have Netflix streaming, I suggest watching "All In This Tea" which is a very interesting documentary about an eclectic tea maker's adventures in China.