Green tea leaves, which preserve the green, fresh flavor, are the least processed of teas. They are steamed or pan-fired just long enough to halt active enzymes in the leaf.
Black teas are another story. Here, the leaves are spread out and allowed to wither. They are then rolled, which bruises the leaves and causes them to release their characteristic flavor. The leaves are then allowed to undergo fermentation, changing color from a dull green to a coppery red to a deep brown and then to a nearly black color. The leaves are then fired or dried in hot ovens.
Oolongs are the specialty of mainland China and Taiwan. Always made from whole tea leaves, they're allowed to wither and then ferment. After about 1 1/2 to 2 hours, the leaves are pan-fired to stop enzymatic action.
White tea, a rare variety, is made from very small buds and leaves that are picked in the early spring just before they open. They are withered to allow the natural moisture to evaporate and then dried, yielding a very delicate, pale-colored tea.