There are two main types of acne scars:
Hypertrophic or keloid scarring:
These scars are caused when the body produces too much collagen as acne wounds heal, resulting in a mass of raised tissue on the skin's surface.
Atrophic or depressed scarring:
These scars develop when there is a loss of tissue. There are two common types of atrophic scarring. "Icepick" scars are usually small, yet obvious holes in the skin. "Boxcar" scars are depressed areas, usually round or oval in shape with steeply angled sides, similar to chickenpox scars.