Tea is generally categorized into six types: black, green, white, yellow, oolong (sometimes "wulong"), and fermented. These categories are born out of a Chinese understanding of tea (the global 'home' of tea) and are based on how the raw leaves are processed. Some teas specific to a culture or region may fall outside these six categories, and other teas are in-between categories, but overall the six types hold up fairly well as a way of organizing tea.
Chart by Tony Gebely, courtesy of worldoftea.org
To learn about the types of tea, it is important to understand the difference between oxidation and fermentation. Oxidation occurs naturally in organic matter exposed to air and mositure - for example, cut bananas and avocados turning brown. The oxidation of tea leaves begins as soon as they are picked and continues until the process is halted, usually through heat-treatment. This means leaves destined to become low-oxidized teas have to be rushed in for processing soon after they are picked. Fermentation on the other hand involves microbial activity and is the process by which beer and wine are made.
Generally speaking, green teas are unoxidized (less than 15% oxidized), oolongs are partially oxidized (15-85% oxidized), and black teas are fully oxidized (85% or greater oxidized). Factors other than oxidation level distinguish white and yellow tea. The most well known fermented tea is pu'er tea (sometimes "pu erh"), but there are other kinds of fermented tea such as brick tea. Almost all fermented teas get better with age, which is why old pu'er is sought by investors and collectors, with particularly high quality examples fetching eye-watering prices.
White tea is the least processed of all the teas. Named for the downy hair on young buds, white tea is air-dried with no additional processing. The brewed tea can range from barely-tinted to a deep orange, with the lighter brews tending to be subtle and elegant while the darker brews are sweet and woody. White tea tends to mellow over time due to slow oxidation in storage.
Green tea owes its vibrant character to heat-treatment soon after picking which halts the oxidation process. Japanese green teas are sometimes steamed before drying, which breaks down the structure of the leaf, giving a jewel green, cloudy brew and grassier taste. Green teas are best consumed fresh, although with good storage you can still expect two years out of all but the fussiest green teas.
Oolongs are known for their strong fragrance and variety of taste, which can be anything from silky and floral to buttery and toasted. Somewhere between a green tea and a black tea, this diverse category can be made from a large number of cultivars and can be smoked or roasted to add different flavor elements. The brewing and storage of oolong tea depends on whether it is lightly oxidized (around 30%, and therefore closer to a green tea) or heavily oxidized (around 70%, and therefore closer to a black tea). Rolled oolongs (those that come in a ball shape) sometimes benefit from hotter water to help them unfurl than their more leafy counterparts.
Most black teas are fully oxidized and may be fired or unfired (the brown color comes from oxidation and not a roasting process). However, some black teas are closer to green teas in character because they undergo extensive initial drying, which removes enough moisture to prevent full oxidation (however, since they do not go through a heat-treatment phase specifically designed to stop oxidation, they are still considered black teas rather than green teas). This is the case with first flush darjeelings, which have green-tinged leaves and brew a light color. Only black tea should ever be taken with milk, and many prefer it without.
Yellow tea is a rare kind of tea made exclusively in China. There is also a Korean 'yellow tea' which is very different and is one of the rare examples of a regional tea that falls outside the usual six categories. Sometimes thought of as a sub-class of green tea, the process for making yellow tea is the same as for making green except that the freshly picked leaves rest under a damp cloth before being heat treated. This 'smothering' lightly oxidises the tea and gives it a more rounded, sweeter, less grassy taste than a standard green tea.
Pu'er is the most well known type of fermented tea. Tea drinkers are usually divided into those that love and those that loathe its distinctive, peaty taste. Originally compressed into bricks for ease of trade, pu'er can be found both compressed and in loose leaf form, although the former is much more common. Pu'er comes in two kinds: raw and cooked. While some enjoy the sharp taste of young raw pu'er, most raw pu'er is aged for many years before consumption. Cooked pu'er, on the other hand, has gone through an accelerated aging process and is ready for immediate consumption.