There are two basic categories of moisturizer: humectants (which, like glycerin soap, absorb water from the air) and emollients.
Emollients soften skin in part by delivering the goods directly. Unlike a humectant, emollients bring moisture to your skin via the composition of the product itself. Carrier oil, urea and man-made substances such as silicone oils and isopropyl myristate are all emollients that you might see on product labels. One moisturizer will often contain several different types of emollients (and possibly humectants as well).
Emollients consist primarily of one of two things: oil or water. Oil-based emollients are greasier to the touch, are more easily seen on the face after application and stick around longer than water-based emollients. Water-based emollients are easier to apply and make less of a mess.
Dry skin can be the result of a problem with your skin's barrier. When detected by the body, this problem can result in an immune system response that causes inflammation and redness. When you apply a moisturizer that contains an emollient, you are soothing and healing the external barrier layer, which in turn will tell the immune system to relax and the inflammatory response to cease.
One of the main functions of an emollient is to trap moisture in the skin. However, if you don't properly apply an emollient, you may just wind up trapping moisture outside of the skin, leading to more problems with dryness. For this reason, it's important to apply an emollient cream when you already have some moisture on your face, hands or any other part of your body you are treating. A good time to apply an emollient is after bathing. Make sure you haven't completely dried off when you apply the emollient, thus trapping the external water beneath the artificial layer of oil you are applying. Otherwise, you'll be sealing in dry skin and preventing moisture from penetrating the emollient layer.
Emollients not only help moisten your skin and trap moisture within, they also reduce the need to use more potent medications that may cause a negative reaction, especially when used over a long period of time. Your dermatologist will know which course of action is best to treat your dry skin, but proper use of an emollient cream is a safe treatment you can undertake right away, and one that usually delivers results.
For lots more information on emollients and skin care, see the next page.