Green tea is precisely what it says. It is the green leaf tips of the tea plant, camellia sinensis, that have been dried without any cutting or tearing (which encourages the leaf to oxidise or ferment, turn brown and to develop bigger, bolder more acidic flavours through the accompanying development of caffeine.) The whole leaves can be cut after drying and will not then change colour or ferment. Green teas retain many antioxidants that would disappear if they were fermented, but as they have little caffeine content, their main appeal is as a warm or hot drink that relaxes rather than stimulates the system.
Read about the benifits of Green Tea