To avoid acne scars, it is very important to not pop pimples or squeeze and pick at blemishes. Doing so well actually make things worse and increase the chances of scarring. For many people, the minor acne scars left behind are from picking or peeling at acne.
Also, while minor to moderate acne generally do not leave scars behind when unattended to, more severe forms of acne might. Often, leaving more severe forms of acne, such as cysts, untreated will increased the likelihood of scars.
There are two different types of scars left behind by acne. One is hypertrophic scars, which is a hard upraised bump. The other are called pits, which are depressed into the skin.
There are ways to get rid of acne scars once you develop them. Common solutions involves dermabrasion, acid peels, and sometimes also collagen injections.