Acne inflammation may damage deeper skin tissue called the dermis leading to acne scar formation. As opposed to acne marks which are pigmentation there is an actually depression in the skin in scar formation.
Depressed scars can be simplistically categorized into ice pick scars which are narrow and deep, box car scars which are wider and shallower, rolling scars in which the skin looks like it has ripples.
There are also hypertrophic scars and keloids which often look like lumps on the skin.
Depressed scars can be improved with techniques such as sub-cision and fractional laser treatment. Hypertrophic raised scars which develop often on the chest, back, neck and even chin can be treated with steroid injections to flatten them.