Green tea (Camellia Sinensis) is a plant frequently steeped in hot water and drunk as tea. It exerts most of its benefits through water-soluble polyphenols (frequently referred to as catechins) extracted from the leaves, into water, which is then consumed.
It has been implicated in benefiting almost every organ system in the body. It is cardioprotective, neuroprotective, anti-obesity, anti-carcinogenic, anti-diabetic, anti-artherogenic, liver protective and beneficial for blood vessel health. However, the practical benefits are still under investigation and simply because it may affect many things doesn't mean that it's particularly potent.