Green Tea, best known for its grassy vegetal notes and greenish liquor and leaves, is quickly steamed or pan-fired to denature the oxidizing enzymes and preserve the tea's characteristic freshness. While all tea is antioxidant-rich, some speculate that the minimal processing undergone by green tea allows more antioxidants to reach your final cup. Without oxidation, green teas must be steeped more carefully, as they can become bitter if steeped too long or at too hot of a temperature. Never steep green tea with boiling water; near boiling or even cooler will produce much better results.
Black tea is the most intensively processed type of tea. The leaves are allowed to fully oxidize, creating their black color before they are dried, giving black tea more complexity, more astringency and fewer vegetal overtones than are typically found in other teas. Astringency is the "dry mouth" sensation left by tannins in tea, familiar to drinkers of a cabernet sauvignon, or other wine. It is this astringency that pairs so nicely with dairy and sweetener. Achieving the right balance of astringency is one of the leading indicators of quality in a black tea.