Oolong tea is a term used for tea that is semi-oxidized. Green, oolong, and black tea all come from the same plant, Camellia sinensis (the tea plant), the difference between them is their level of oxidation. Green tea is unoxidized, oolong tea is semi-oxidized, and black tea is fully oxidized. Oxidation occurs naturally when cells of the tea leaves are ruptured, thus releasing enzymes that happen to catalyze oxidation of the tea leaves. The cells are most often ruptured by shaking, raking, or rolling the leaves. Oxidation of the tea leaves is stopped by destroying the enzymes. The enzymes are most often destroyed by denaturing them by steaming or pan-firing at high temperature. So if one wants a green tea, one would pick the tea leaves and steam or fire them to prevent any oxidation. If one desired black tea, one would pick the leaves, rupture cells in the leaves to induce oxidation (most often by rolling), let the leaves oxidize for several hours, then fire them to dry the leaves and halt oxidation (often done in large woks or ovens). So if one wants a oolong tea, one would pick the leaves, rupture just a certain percentage of the cells in the leaves to induce slow and controlled oxidation (often by repeated cycles of gently shaking in bamboo baskets and then allowing them to slowly oxidize and dry in the shade), achieve the desired level of oxidation ( can vary from 20% for a greener oolong to 60% for a classic Formosa (Taiwanese) oolong), then quickly fire them to dry the leaves and halt oxidation (often pan-fired). Preparing oolong teas is obviously more time consuming and difficult.
Because oolong tea is semi-oxidized, it has taste characteristics of green and black teas, but it also has flavors of its own. I would encourage anyone who appreciates tea to try a few oolongs. We offer several at the shop and we'd be happy to help you select from among them.