same as all other tea, camellia sinensis. It is called an antioxidant because it contains antioxidant compounds. Most notibly EGCG (epigallocatchin - 3 - gallate) which protects most notibly against tumors, and the sheer ammount of antioxidants that assist to prevent aging by turning a free radical propagation reaction into a termination.
There are four advantages to green tea... Its beauty, its taste, its aroma, and its health benefits. Learn to enjoy the first three and you'll forget you drank it for the fourth. ^^