Stretch marks are a form of scarring on the skin with a silvery-white hue (when they are new they can appear pink, red or a purple colour). They are caused by tearing of the dermis and over time can diminish but not disappear completely.
Stretch marks are the result of the rapid stretching of the skin associated with rapid growth (common in puberty) or weight gain (e.g. pregnancy) that overcomes the dermis' elasticity. Although stretch marks are generally associated with pregnancy and obesity, they can also develop during rapid muscle growth. They first appear as reddish or purple lines, but tend to gradually fade to a lighter colour. The affected areas appear empty and soft to the touch.
Stretch marks occur in the dermis, the resilient middle layer that helps the skin retain its shape. No stretch marks will form as long as there is strong support within the dermis. Stretch marks can appear anywhere on the body, but are most likely to appear in places where larger amounts of fat are stored. Most common places are the abdomen (especially near the belly-button), breasts, upper arms, underarms, thighs (both inner and outer), hips, and buttocks. They pose no health risk in and of themselves, and do not compromise the body's ability to function normally and repair itself.
Stretch marks located in the most inconvenient locations can ruin your best efforts at looking good. They can dramatically affect how you feel about yourself and your appearance; especially at summertime when you head for the beach.