Green tea gets it's color and name by the way the leafs are grown, plucked and processed. When withered and heated the tender tea leaves start to turn green and the longer the heat and rolling of the leaves the darker green they will be. These factors along with climate and when leaves are plucked help create many green tea varieties.
Most green teas come from China or Japan. The main difference is that Chinese green teas are pan fired giving a slightly earthy nutty taste while Japanese green teas are steamed and may have a bit of grass like notes.