Made from the leaves of Camellia sinensis, black tea is the most oxidized* tea variety and is generally stronger in flavor than oolong, green, and white teas. It accounts for over 90% of tea sold in Western cultures.
Oolong is a traditional Chinese tea that comes from Camellia sinensis leaves that are semi-oxidized. Most oolong teas come from unique cultivars that are used exclusively for particular varieties, and taste can vary widely - from sweet and fruity to woody and roasted to fresh and floral.
Green tea is produced from Camellia sinensis leaves that have undergone very little oxidation. Strongly associated with Asian culture, it has become more widespread in the West due to its potential for beneficial health effects, which are still being evaluated.
Buds and leaves of the Camellia sinensis plant are used to produce subtle-flavored white tea by undergoing very little oxidation.
Pu-erh is a variety of post-fermented teas known as Hei Cha (teas that undergo microbial fermentation after they are dried and rolled). It is often sold pressed into various shapes, such as a disc, brick, or square, and is generally served Gongfu style (in a traditional Chinese tea ceremony).
Herbal tea is not a "true" tea, but rather refers to any non-caffeinated beverage made from steeping herbs, spices, flowers, and other plant materials in hot water. Also called "tisanes", herbal teas are often consumed for their medicinal effects (such as drinking chamomile tea for its sedative effect).
Blended tea is tea (often black tea) blended with other teas or plant material to produce a desired flavor. Blended teas are often classified as "breakfast" (robust, full-bodied) or "afternoon" (lighter than breakfast blends).
*Oxidation refers to the chemical process by which tea leaves become brown and produce their distinctive flavor and aroma compounds. This is sometimes incorrectly referred to as "fermentation" - a process which describes chemical breakdown by microorganisms, which is not present in most teas (other than post-fermented teas like pu-erh).