From Angela Palmer,
Your Guide to Acne.
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About.com Health's Disease and Condition content is reviewed by Casey Gallagher, MD

Acne vulgaris, or common acne, is a disorder of the pilosebaceous unit, causing some form of lesions on the skin. Acne is characterized by the presence of one or more of the following:

• papules
• pustules
• blackheads
• whiteheads or milia
• nodules
• cysts

Non-Inflamed Acne

Acne can be divided into two categories: inflamed and non-inflamed. Non-inflamed acne breakouts consist of microcomedones, blackheads, and milia. These types of comedones are not red or painful. Symptoms of non-inflamed acne include bumps or bumpiness across the skin's surface, or an uneven skin texture. Even if comedones are not readily visible, they will make the skin feel rough or "sandpapery."

People with non-inflamed acne experience blackheads, milia, and closed comedones but rarely have reddened breakouts, such as papules or pustules. Left untreated, non-inflamed acne may progress to inflamed acne.

• Types of Non-Inflamed Acne Blemishes
• Acne Cosmetica
• Smoker's Acne

Inflamed Acne

Inflamed acne is characterized by redness and inflammation. Those with inflamed acne will have microcomedones, blackheads, and milia, as well as papules, pustules, and possibly nodules and cysts. Symptoms also include redness, swelling, and irritation of the skin, along with possible crusting, oozing, or scabbing of the lesions.

Inflamed acne ranges in acuity from very mild to extremely severe. Some inflamed acne sufferers will experience only the occasional pustule while others will battle angry-looking cysts. Cystic acne is the most serious form of inflamed acne. Inflamed acne sufferers are most at risk for skin damage and scarring.

• Types of Inflamed Acne Blemishes
• Cystic Acne

Range of Acne

The areas of the body most typically affected by acne are:

• face
• neck
• chest
• shoulders
• upper arms
• back

Less commonly, acne appears on the torso, arms, and legs.

• Stages of Acne
• Back and Body Acne

Pigmentation and Scarring

Most acne sufferers have to deal with at least some degree of pigmentation problems or scarring. Discolored, darkened, or reddened spots or blotches are common on acne prone skin. These problems can persist even after breakouts have fully healed.

Post-inflammatory hyperpigmentation (PIH) is the medical term used to describe discolored spots (macules) left behind after an acne lesion has healed. Those who have any type of inflamed breakouts are at risk for post-inflammatory hyperpigmentation. Luckily, PIH is not a true scar and will fade over time.

More serious scars include:

• ice pick scars
• rolling scars
• atrophic (depressed or pitted) scars, and boxcar scars
• hypertrophic (raised) scars
• keloids

The more severe the acne, the higher the chances of developing these types of acne scars.

• What Causes Acne Scars?
• Types of Acne Scarring
• Prevent Acne Scars
• Acne Scar Treatments