It's noted for it's polyphenols, present in the fresh leaves. Many of these polyphenols are destroyed when tea is allowed to ferment as in the black teas, but because it is processed as little as possible, green tea retains most of it's polyphenol content. Polyphenols are a group of naturally occurring plant chemicals with antioxidant potential. Antioxidants protect our bodies from free radicals which are highly reactive molecules and molecule fragments that cause damage at a cellular level. This sets the stage for cancer and many of the degenerative diseases.
Antioxidants deactivate free radical damage and protect our bodies from disease.
Polyphenols are in their highest concentration in green teas, but also in apples, grapes and onions, as well as other plants. The 4 polyphenols in green tea are collectively called catechins. Green tea does contain caffeine and how you make your tea affects its caffeine level. When you brew tea you get between 20 - 90 milligrams of caffeine. Instant or iced tea has less.