There are three main varieties of tea - black, oolong, and green. The difference between the teas is in their processing.

Black tea undergoes a fermentation process, which oxidizes the leaves and turns them "black". This process deactivates many of the antioxidants called polyphenols, and creates a bitter taste. Oolong tea is oxidized about half as long (semi-fermented) and is then steamed to stop oxidation, and has bitter taste. Green tea is not fermented at all. It is steamed to stop oxidation before it starts, which preserves most of the polyphenols. Green tea has 60% more polyphenol (antioxidants) content than black tea, and has a less bitter taste.

Polyphenols can help the body protect itself against free radicals; molecules, which occur in the environment and are naturally produced by the body, and can cause damage to cells. Chronic damage by free radicals is one factor thought to contribute to the development of many chronic diseases including heart disease or cancer. Polyphenols scavenge cell-damaging free radicals, which are linked with cancer-causing genes and cause LDL cholesterol to form artery-clogging plaque. The polyphenols in tea possess 25 to 100 times the antioxidant potency of vitamins C and E. Antioxidants impair the ability of free radical cells to harm the molecules that make up our bodies.

The polyphenols that exist in green tea are mainly (-)-epigallocatechingallate (EGCG), (-)-epigallocatechin (EGC), (-)-epicatechingallate (ECG), (+)-epicatechin (EC), (-)-gallocatechingallate (GCG), and (-)-catechin. EGCG is the most powerful and the most abundant of the six major catechins found in green tea.