Plant extract that contains salicin, a substance that when taken orally is converted by the digestive process to salicylic acid (beta hydroxy acid).
The process of converting the salicin in willow bark into salicylic acid requires the presence of enzymes, and is complex. Further, salicin, much like salicylic acid, is stable only under acidic conditions. The likelihood that willow bark in the tiny amount used in cosmetics can mimic the effect of salicylic acid is at best questionable, and, in all likelihood, impossible. However, willow bark may have soothing benefits for skin.
References for this information:
Phytotherapy Research, August 2015, pages 1,112-1,116; and July 2011, pages 1,102-1,104
Phytomedicine, December 2010, pages 1,106-1,113