Definition: Tea is a beverage made by steeping processed leaves, buds, or twigs of the tea bush, Camellia sinensis, in hot water for a few minutes. The processing can include oxidation, heating, drying, and the addition of other herbs, flowers, spices, and fruits. The four basic types of true tea are (in order from most to least processed): black tea, oolong tea, green tea, and white tea. The term "herbal tea" usually refers to infusions of fruit or of herbs (such as rosehip, chamomile, or jiaogulan) that contain no Camellia sinensis . (Alternative terms for herbal tea that avoid the word "tea" are tisane and herbal infusion.) This article is concerned exclusively with preparations and uses of the tea plant C. sinensis.

Tea is a natural source of the amino acid theanine, methylxanthines such as caffeine and theobromine, and polyphenolic antioxidant catechins. It has almost no carbohydrates, fat, or protein. It has a cooling, slightly bitter, and astringent flavor.

 

Processing and classification
Tea processing

Tea plant (Camellia Sinensis) from Köhler's Medicinal Plants.These types of tea are distinguished by the processing they undergo. Leaves of Camellia sinensis soon begin to wilt and oxidize if not dried quickly after picking. The leaves turn progressively darker because chlorophyll breaks down and tannins are released. This process, enzymatic oxidation, is called fermentation in the tea industry although no true fermentation happens (that is, the process isn't microorganism-driven). The next step in processing is to stop the oxidation process at a predetermined stage by heating, which deactivates the enzymes responsible. With black tea this is done simultaneously with drying.

Without careful moisture and temperature control during its manufacture and thereafter, fungi will grow on tea. This form of fungus causes real fermentation that will contaminate the tea with toxic and sometimes carcinogenic substances and off-flavours, rendering the tea unfit for consumption.

Tea is traditionally classified based on producing technique :

White tea
Young leaves (new growth buds) that have undergone no oxidation; the buds may be shielded from sunlight to prevent formation of chlorophyll. White tea is produced in lesser quantities than most other styles, and can be correspondingly more expensive than tea from the same plant processed by other methods. It is less well known in countries outside of China, though this is changing with increased western interest in organic or premium teas.
Green tea
The oxidation process is stopped after a minimal amount of oxidation by application of heat, either with steam, or by dry cooking in hot pans, the traditional Chinese method. Tea leaves may be left to dry as separate leaves or they may be rolled into small pellets to make Gunpowder tea. This process is time consuming and is typically done with pekoes of higher quality. The tea is processed within one to two days of harvesting.
Oolong
Oxidation is stopped somewhere between the standards for green tea and black tea. The oxidation process takes two to three days. In Chinese, semi-oxidized teas are collectively grouped as blue tea (青茶, literally: blue-green tea), while the term "oolong" is used specifically as a name for certain semi-oxidized teas.
Black tea/Red tea
The tea leaves are allowed to completely oxidize. Black tea is the most common form of tea in southern Asia (Sri Lanka, India, Pakistan, Bangladesh, etc.) and in the last century many African countries including Kenya, Burundi, Rwanda, Malawi and Zimbabwe. The literal translation of the Chinese word is red tea, which is used by some tea lovers. The Chinese call it red tea because the actual tea liquid is red. Westerners call it black tea because the tea leaves used to brew it are usually black. However, red tea may also refer to rooibos, an increasingly popular South African tisane. The oxidation process will take between two weeks and one month. Black tea is further classified as either orthodox or as CTC (Crush, Tear, Curl, a production method developed about 1932). Unblended black teas are also identified by the estate they come from, their year and the flush (first, second or autumn). Orthodox processed black teas are further graded according to the post-production leaf quality by the Orange Pekoe system, while CTC teas use a different grading system.
Post-fermented tea
Teas that undergo a second oxidation, such as Pu-erh, Liu'an, and Liubao, are collectively referred to as secondary or post-fermentation teas in English. In Chinese they are categorized as Dark tea or black tea. This is not to be confused with the English term Black tea, known in Chinese as red tea. Pu-erh, also known as Póu léi (Polee) in Cantonese is the most common type of post-fermetation tea in the market.
Yellow tea
Either used as a name of special tea processed similarly to green tea, or high-quality tea served at the Imperial court.
Kukicha
Also called winter tea, kukicha is made from twigs and old leaves pruned from the tea plant during its dormant season and dry-roasted over a fire. It is popular as a health food in Japan and in macrobiotic diets.