Different types of tea are distinguished by when the leaves of the tea plant were harvested and how long the leaves were allowed to age before being dried.

All true tea (green, black, oolong, white and pu-erh) comes from the leaves of the tea plant,  Camellia sinensis.

What separates the different varieties is how the leaves were processed.

 
White tea is harvested early, before the leaf buds have even opened fully. It is then air-dried quickly, producing tea with a delicate flavor. All other tea is produced from mature leaves.

 
Green tea leaves are dried immediately after harvesting, before they can begin to oxidize, while leaves for black tea are allowed to sit out and oxidize (a process often incorrectly called fermentation) before drying.

Oolong tea is produced from leaves that were not oxidized as long as black tea leaves. These different processes explain the greener, more plant-like taste of green tea and the intermediate flavor of oolong tea.
A fifth type of tea, pu-erh, actually consists of two varieties. Raw pu-erh is a variety of green tea that ages well and need not be used quickly, while ripened pu-erh is made from green tea leaves that have truly been fermented.

 

Sources:

http://www.imperialteagarden.com/teas.html
http://en.wikipedia.org/wiki/Puer_tea

 

Extract from 25 Amazing and weird facts about how food is made, and were it comes from, by Mike Adams and Daivd Guiterrez, who are the editors of www.NaturalNews.com