Willow bark is a source of natural hydroxy acids that gently exfoliate.
The extract coming from the bark of the White Willow, a big (25 m/80 ft.) tree that likes to live on riverbanks. It's famous for containing anti-inflammatory natural salicylates (this powder, for example, is standardized to contain 53-65%), a close chemical relative to famous exfoliant salicylic acid.
Thanks to its salicin content, willow bark is often touted as a natural alternative to salicylic acid, though it's quite questionable how effective it is as a chemical exfoliant in the tiny amounts used in cosmetics. Apart from soothing salicin, it also contains flavonoids and phenolic acids that give willow bark tonic, astringent, and antiseptic properties.