Green tea and black tea is made from the same bush/plant called Camellia Sinensis. It is the method of manufacturing that makes it different.
The tea is plucked and taken to the factory wherein immediately leaves are steamed or pan fired. This prevents oxidation (turns brown). Hence the color, taste, aroma of the tea is close to how it was when it was plucked.
In comparison, the black tea is an oxidized tea. The leaves are plucked, taken to the factory. The tea leaves undergo a process wherein the color, aroma, taste changes due to oxidation. Hence it is black in appearance and dark brown in the cup.