Green tea is a type of tea that is harvested and then quickly preserved. Whereas black tealeaves are allowed to oxidize after they are picked, green tealeaves are immediately heated to prevent oxidation. (Oxidation is a natural process. It's the same thing that happens when you slice an apple and it begins to turn brown and taste sweeter as it is exposed to oxygen.)
Green teas are processed with either steam heat or with dry heat (such as pan firing, which is similar to stir frying in a wok, or a quick baking process in an oven).