Herbal tea is not actually tea, but rather an herb or a mix of herbs. Only in the United States are these herbal mixtures called tea. They are usually referred to as an infusion or tisane in other parts of the world. Medicinal teas are tisanes made from specific herbs, flowers and extracts that may be beneficial. True teas have been widely studied for their health benefits.
Green tea contains polyphenols, researched for their antioxidant and other healthy properties.
Both green and black tea come from the Camellia sinensis plant. The difference is in the amount of time that the picked leaves are allowed to oxidize. Green teas are minimally oxidized, whereas Black teas are fully oxidized. The industry term for what goes into tea bags is 'fannings' or 'dust'. Only the smallest particles of tea, trailings of mass production, make it into tea bags.