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 indeed have some skin-soothing benefits for skin. 
- Vlachojannis J, Magora F, Chrubasik S. Willow species and aspirin: different mechanism of actions. Phytother Res. 2011;25(7):1102-4.