Green teas are unoxidized teas, that due to heating up they have preserved their chlorophyll and their green, fresh taste. The heating up process closes the leaf's veins and stops any oxygen from coming in to the leaf. Heating can be made either by pan-firing the leafs (often referred as the Chinese way) or by steaming (often called the Japanese way). The pan-firing takes about 2-3 minutes and creates nutty, toasty and ripe flavours in the tea. The steaming is much quicker, about 20-80 seconds, and that's why the steamed green teas have more fresh flavours, the leaves are greener and have more grassy, vegetal and mineral flavours when compared with pan-fired green teas.
Green teas are sensitive towards too high brewing temperatures, so brew your green teas with utmost care!