There is only one Beta Hydroxy Acid. It's Salicylic Acid. It works mainly as an exfoliant causing the skin cells to become loose. This allows the dead skin cells to slough off and makes room for new cells. It is reported to improve wrinkling roughness and uneven pigmentation after 6 months of use. It works best in concentrations of 1% - 2%.
The main difference between alpha hydroxy acids (AHA's) and beta hydroxy acids is that it is oil soluble. Salicylic 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, salicylic acid is better used on oily skin.
AHA's work better on sun damaged skin where break outs are not a problem. Salicylic Acid can increase sun sensitivity by as much as 50%. So on one hand it helps sun damage, but on the other hand, it increases photo sensitivity.
It is less irritating than AHA's even though it penetrates deeper into the pores. This is because it is derived from aspirin, which has anti-inflammatory properties. Even so, it can still cause irritation including redness, burning and itching.
According to the FDA, the safety of salicylic acid used as a cosmetic ingredient has been evaluated by both the cosmetic industry and FDA. At a meeting in February 2000, the Cosmetic Ingredient Review (CIR) Expert Panel, the cosmetic industry's independent body for reviewing the safety of cosmetic ingredients, reached the tentative conclusion that the use of salicylic acid related substances in cosmetics is "safe as used when formulated to avoid irritation and when formulated to avoid increased sun sensitivity." CIR added that "when sun sensitivity would be expected, directions for use [should] include the daily use of sun protection."
In other words, it's safe, but be sure to use a sunscreen when using it!
Salicylic acid is found in a variety of skin care products including moisturizers, cleansers, eye cream, sunscreen, and foundations.