AHAs hydrate the surface of the skin and are water soluble. Examples of AHAs are glycolic and lactic acid.
These types of acids are best used on dry, thick, sun-damaged skin.
BHAs are drying to the surface of the skin and are oil (lipid) soluble. This means that BHAs are able to penetrate into the pore which contains sebum (oil) and exfoliates the dead skin cells that are built up inside the pore.
Therefore, individuals with oily or acneic skin would benefit by using this type of acid. Salicylic acid is a BHA.