Coffee? PHLEUGH. I drink tea. I drink it because it's delicious and healthy. I drink it because it's cheap, diverse, and doesn't make my breath stink. But this überdrink has a secret. Tea, you see, is not as it seems.
Tea is a drink...
More specifically, tea is an infusion: a bunch of dried leaves steeped in water—usually hot—which releases all kinds of great stuff, including vitamins (E, C), caffeine and antioxidants. These combine for an agreeable flavor.
...that is wildly popular in most of the world...
As an American, it's possible—though not easy—to forget about tea. After all, the average American drinks less than half as much tea as a typical Western European, and about one twelfth as much as your average Briton.
The figures make sense. In America, tea has long since abdicated its role as Morning Hot Thing with Caffeine to coffee. The American conception of tea tends to involve ice.
...and all comes from a single leaf...
Tea has diversity in its favor—you can buy green tea and enjoy a soothing gulp with a distinctly plant-y finish, or opt for a cup of black tea for something a bit more bold and caffeinated. Here's the thing, though: Black, white, green, yellow, Oolong, and Pu-erh may look, smell and taste different, but they're basically all the same. At least in terms of where they come from. Say hello to Ms. Camellia sinensis, mother of all tea:
When people talk about "tea leaves", they're talking this here plant, a flowering member of the shrubby Theaceae family. Some teas may call for Camellia sinensis leaves from different regions of the world, where climate and soil composition affect flavor, but all tea is, by definition, from this plant. (Herbal "teas" are really just infusions or decoctions of other plants. They're like tea, but they're not tea.)
That said, camellia sinensis leaves—wherever they're from—don't become true tea leaves until they've been somehow treated or cured.
...treated in a variety of ways...
Between leaf and cup, quite a bit has to happen. The basic process runs something like this: small leaves and buds are picked from the plant, and laid out to dry—either from exposure to a heat source or from ambient temperature. Various secondary treatments then follow. These "secondary" treatments are the source of everything unique about a given type of tea, and although they can be complicated, they work from a few basic principles:
Oxidation: If left picked and undried, tea leaves will naturally begin to oxidize, through a process known as enzymatic browning, a process in which an enzyme—polyphenoloxidase—catalyzes reactions in various substances in the tea, called phenolic substrates. (One of these substrates is tannin.) In some lighter teas, like white tea, oxidation is stunted by briefly cooking the leaves, which deactivates the enzymes responsible for the color change. Where oxidation is desirable, like in black tea, the leaves are crushed otherwise wounded, which increases the rate of browning. Oxidation is often incorrectly referred to as fermentation, which implies some kind of microorganism involvement, which, EW.