Green teas are "unfermented," that is, they are not oxidized at all—they have a fresh, almost grassy flavor, sometimes with a licorice-like undertone.
White teas, uncomplicated and sweet, are air-dried and are partially oxidized. The best quality white teas are picked before the leaf-buds have opened, while they're still covered by silky white hairs—hence the name.
Oolong teas are partially oxidized before being fired to stop the enzymatic action. Some are very lightly oxidized, others much more heavily so. They have a complex, mellow flavor, with a slight bitterness at first sip but a long, sweet finish. Because the term "oolong" can refer to a tea plant and a means of processing as well as to the finished tea, Chinese and Hong Kong tea people are replacing the word with "semifermented"—even though the teas are actually oxidized rather than fermented.
Black teas, which are fully oxidized, are rich and fruity, with a stronger flavor than the other types.