Because true teas all come from the same plant, Camellia sinesis, the variation between them is largely determined by how much the raw tea leaves are fermented or oxidized. Fermentation is actually a misnomer as no sugars in the tea are converted into alcohol.  The more proper term is oxidization, which describes the reaction of enzymes in the tea leaves, which oxidize when exposed to the open air during the rolling process after the leaves are withered.  This process is similar to an apple which has been cut open. When the meat of the apple is exposed to air it eventually turns brown because of exposure to the open air.

As noted, black teas are fully oxidized; oolong teas are semi-oxidized; while green and white teas are not oxidized at all.

The process of oxidization can be arrested by the drying the tea leaf. Chinese orthodox teas were traditionally dried in a hot wok. Picking the right moment to stop oxidization is particularly important with oolong teas because oxidization must be arrested before they fully oxidize to impart oolong’s unique flavor.