Tea is the second-most-consumed beverage in the world after water. All true teas come from the Camellia sinensis,
and the different "colors" of tea are created by processing the tea leaves in different ways. After tea leaves are picked, they are laid out for between 8 and 24 hours to dry; then the real processing begins.
Tea leaves' veins contain enzymes, and when those enzymes are exposed to air, they cause oxidation, or fermentation. Whether, how, and for how long a tea leaf is allowed to oxidize changes the look and flavor of the tea it creates.
Black tea is fully oxidized. After the tea leaves are dried, they are rolled so that the surface of the leaves is broken up and the enzymes are released. The leaves are then left to fully oxidize, and in the process, they turn black. Black tea contains more caffeine than the other colors of tea.
Green tea is not oxidized. After the leaves are dried for a bit, they are steamed or pan fried to keep any oxidation from occurring. They are then rolled in various ways and tightnesses and go through a final drying. Because they are unoxidized, green tea leaves retain most of their original green color. They also contain less caffeine than black tea.
White tea is the least processed of the teas. After a short period of drying, they are immediately fired or steamed to prevent oxidation. They are not rolled, bruised, cracked, or broken in any way. White teas have the least amount of caffeine and possibly the most antioxidants. Because they are relatively unprocessed, white tea tastes more like fresh leaves than any of the other teas.
Although oolong doesn't seem like a color, this final true tea's name actually means "black dragon" in Chinese. Oolong tea is the most difficult to process. It is only partially oxidized and falls somewhere between black tea and green tea in caffeine and color. After a brief drying period, the tea leaves are tossed in baskets to bruise the edges, releasing some but not all of the enzymes to result in partial oxidation. The leaves are then steamed (to stop the oxidation) and dried.
Red tea isn't really a tea at all, but an herbal infusion, because it doesn't come from the tea plant Camellia sinensis. It comes from the Aspalathus linearis, otherwise known as Rooibos or "red bush." The Rooibos is a delicate plant from South Africa that is difficult to harvest, and it is processed in a way that is very similar to the way tea leaves are processed. Rooibos leaves are originally green, but they take on a red color during processing. Red tea has no caffeine.