Salicylic acid is a beta hydroxy acid that it is used mainly as an exfoliant. It is a keratolytic, which means that it works by softening the outer layer of the skin allowing it to loosen and shed. It causes the cells of the epidermis to become "unglued" allowing the dead skin cells to slough off, making room for re-growth of new skin.
Beta hydroxy acid is reported to improve wrinkling, roughness, and mottled pigmentation of photodamaged skin after at least 6 months of daily application. Beta hydroxy acid found in skin-care products works best in a concentration of 1% to 2% and at a pH of 3 to 4. Salicylic acid is used for a number of different skin conditions caused by thickened, hard skin, such as warts, psoriasis, scaly skin conditions, and some nail infections.
The main difference between alpha hydroxy acids and beta hydroxy acid is their lipid (oil) solubility. Alpha hydroxy acids are water soluble only, while beta hydroxy acid is lipid (oil) soluble. This means that beta hydroxy acid is able to penetrate into the pore which contains sebum and exfoliate the dead skin cells that are built up inside the pore. Because of this difference in properties, beta hydroxy acid is better used on oily skin with blackheads and whiteheads. Alpha hydroxy acids are better used on thickened, sun-damaged skin where breakouts are not a problem.