A detailed and comprehensive discussion of acne scars starts with causes of scarring and types of scars, before we explore the prevention and treatments for scars.

Acne leaves scars like any other injury to the skin. Severe cystic and nodular acne always leaves a scar while in some people mild inflamed acne also leaves scars. As we age, these scars become more visible because of loss of collagen in the skin.
When acne resolves, a dark spot forms on the skin. Most often it is PIH (Post Inflammatory Hyper-pigmentation). This dark color of skin is the result of natural body repair. PIH fades away in about a year. If PIH does not resolve during that period, you may have an acne scar that will need treatment.

Raised acne scars
Acne scars can be either depressed or raised. Raised acne scars are formed in those people who develop keloids. Keloid is a growth of tissues at the site of injury. It is not easy to remove keloids by surgery. That may result into more new keloids.

Keloids are more commonly found on male torsos. Its possible treatments includes surgical removal, injections with steroids, cryotherpy, creams and gels. Other type of increased tissue formation scar is called hypertrophic scar. Hypertrophy means overgrowth.

Depressed acne scars
The depressed acne scars form as a result of loss of tissue at the acne site  when the acne spreads inside the skin. Some collagen is lost in this process. After acne heals, the depression remains and new skin grows over the depression.

These depressed acne scars may be classified into:

Icepick scars

Icepick scars are commonly deep scars. They may also be superficial. They look sharp as if the skin has been cut with an icepick. These scars are difficult to treat with laser or dermabrasion as they go deeper inside.

Boxcar scars
These scars do not have sharp edges unlike icepick scars. Boxcar scars are oval or round and commonly they are not very deep.

Rolling scars
When the scarred skin looks rolled, the scars are known as rolling scars.

Depressed acne scars can be removed with surgical and physical procedures such as dermabrasion, chemical peeling, microdermabrasion, surgery, soft tissue filling, laser, etc. Some topical formulations are also available to reduce the intensity of acne scar and lighten it.

(*Summary about Acne Scars by Cdmohatta)