Green tea extracts rich in EGCG (epigallocatechin gallate) are thought to increase calorie and fat metabolism.  Antioxidant polyphenols (epigallocatechin gallate or EGCG, epigallocatechin or EGC, epicatechin gallate or ECG) are thought to be responsible for protection against atherosclerosis and heart disease.

EGCG is thought to be responsible for reducing risk of cancer. Possible mechanism is prevention of new blood vessel growth in tumors (so essentially you starve the tumor). 

Polyphenols in green tea are thought to produce its antidiarrheal effects.

Caffeine in green tea may prevent adenosine’s inhibition of dopaminergic transmission. Thus allowing for more dopamine which results in decrease in clinical symptoms of Parkinson’s disease.