Glycolic acid is considered a natural fruit acid (it comes from sugar cane) as well as an alpha-hydroxy acid. It is used to remove heavy rust and grease from metal materials, but it is better known for reducing the buildup of dead skin cells.
In addition, it draws moisturizers into the skin and is commonly recommended for use in combination with a toner and cream to create an even, smooth, and more youthful-looking complexion. It often creates a tingling sensation on skin, but it shouldn't be painful upon application.
There is a bit of controversy surrounding glycolic acid because of its potential dangers. (Remember, it's also used to remove rust from metal!) If the concentrations are too high, it can cause serious burns. Also, skin becomes sensitive to sunlight, so a high SPF is necessary when going outside. Have you ever tried any products containing glycolic acid? If so, were the results worth it?