I've often seen panthenol in the ingredients of shampoos, conditioners and moisturizing facial products. Panthenol can be classified as a humectant, emollient and moisturizer. A humectant draws water to a surface (for our purposes hair and skin) thus increasing hydration. An emollient makes skin soft and supple and generally results in a soothing sensation.
Panthenol easily binds to the hair follicle and seals the surface which increases shine (Remember the post about shine and scatter?). This is the ingredient that Pantene capitalizes on... I've seen it in every one of their products that I've tried (thats almost all of them! I love Pantene). In hair treatments it is used in a concentration of 0.1-1%. The skin can absorb panthenol rather easily and effectively. It can be used to treat sunburn, minor burns and certain skin conditions. I found it in my moisturizing toner (I swear by toner!).
I guess after all that I should tell you guys exactly what it is. Panthenol is the alcohol analog of pantothenic acid (also known as vitamin B5!). In lay-man's terms, a piece of the B5 chemistry has been replaced with an alcohol. So they're kind of like cousins. Panthenol has passed the CIR (Cosmetic Ingredient Review) for safety in concentrations up to 25%. I haven't read any negative press about this ingredient, but if I find any, rest assured I'll report it.