Catechins are a type of flavonoid, also called condensed tannins, making up 10% to 18% of the polyphenols in green tea. (We also find hydrolyzable tannins like procyanidins.) They offer anti-oxidizing features – they have been shown to be more effective than BHT, which is incredibly effective – as well as anti-microbial and anti-fungal properties.
There are 6 main catechins found in green tea, but the most exciting is epigallocatechin gallate (EGCg or EGCG), which is probably the most studied and the most active polyphenolic. (The others are catechin, gallocatechin, epicatechin, epigallcatechin, and epicatechingallate).