All types of tea - green tea, oolong tea, black tea - come from the tea plant, Camellia sinensis. The differences between these three classes of tea comes from the way the tea is processed after the leaves are picked. Green tea is an unoxidized tea, oolong tea is partially oxidized and black tea is fully oxidized.
Oxidation is a natural process which causes vegetation to turn dark after it is picked. Green tea is produced by steaming the tea leaves shortly after they are picked. Steaming halts the oxidation process, resulting in dried tea leaves which retain their bright green color.
Oolong tea is a partially oxidized. After picking, the tea leaves are tossed in large bamboo baskets to bruise them slightly. Afterwards, they are exposed to the air for several hours which begins the oxidation process. Oxidation is halted by pan roasting the leaves.
Camellia sinensis is the species which produces all types of tea, but within that species there are plenty of varieties (or cultivars or varietals).
The marvellous plant which produces all the world's tea has more than 3000 hybrids. These are referred to as varietals - a term borrowed from the world of wine grape cultivation.
Tea varietals are developed with an aim to producing a particular type of tea in a particular locale. Varietals suited for the production of Indian black tea, for example, do not necessarily make a good Japanese green tea.
Taiwan has been producing oolong tea for about 200 years. During that time tea farmers and scientists have developed varietals which are well-suited to oolong tea production in the climate and soils of Taiwan.
Some of the most common Taiwan varietals are:
Qing Xin Oolong (Green Heart) - Originates in Taiwan. At one time Qing Xin was used in 40% of Taiwan's tea plantations. It is a small dense tea bush and the leaves have pronounced veins. It is used for Oolong tea and Bao Zhong tea. It is more prone to disease than other Taiwan varietals.
Qing Xing Da You - Another Taiwan varietal that is more popular than Qing Xin Oolong. Tea farmers favor Da You because of its greater leaf production and low maintenance. Da You is a short tea bush that is disease-resistant. The leaves are oval-shaped with a blunt tip and serrated edges. They have a large central vein with less-obvious side veins. This varietal is suited to Bao Zhong and Oolong teas.
Taiwan #12 - Also known as Jin Xuen. This varietal has light-green, oval-shaped leaves which are bigger than Qing Xin or Da You. It is resistant to a wide variety of diseases and pests, and produces a greater yield than other varietals. It has a fragrant taste profile which is suitable for both Bao Zhong and Oolong teas.
Taiwan #13 - Also known as Cui Yu (Green Jade). Originates in Taiwan. Grows as a loosely-formed bush which is suitable for hand-harvesting. Has a slightly lower growth rate than Taiwan #12 but with a very desirable floral aroma and taste.