Green teas are the most ancient teas. Originally from China, they were also transplanted to Japan many centuries ago.
Green tea production methods vary but the focus is always to preserve the green color, thus, green teas are not oxidized (read on oxidation HERE). The leaves are heated soon after picking in order to stop oxidation. This type of processing preserves a high level of antioxidants, vitamins & minerals accounting for the various health benefits of green tea.
Higher grades of green tea have a fuller, more complex flavor, & can usually be steeped more times than the lower grades.
The lack of oxidation is responsible for the very low caffeine content of green tea. Its caffeine effect produces a nearly steady, mild high with no big peaks or plunges. It is therefore the perfect meditative aid because it acts as a mild stimulant, without causing insomnia or nervousness. It refreshes & quiets.