Green tea and black tea both come from the same plant called Camellia Sinensis. The difference lies in the way the tea leaves are processed after harvesting.
When making black tea, the tea leaves are crushed. The resulting plant sap is then subject to an enzymatic reaction, oxidation and, in some cases, microbial fermentation. The oxidation promoted by heat changes the color of the cell juices from green to red. The chlorophyll breaks down. Brown pigments are produced which cause the leaves to darken and the leaves become coppery red. At the same time, the tea aroma changes through enzymatic reactions with the flavoring substances it contains.
When making green tea, the tea leaves are also broken up. However, the enzymatic and oxidative processes are prevented by the supply of heat. Due to the lack of oxidation of the cell juices, the tea leaves remain green. Secondary plant substances such as epigallocatechin gallate and aromatic amino acids are retained and lead to a characteristic green tea aroma.