Blemishes are called acne lesions in the medical community. The simplest of acne lesions is a comedo — a plugged hair follicle.

A whitehead is a closed comedo, a plugged follicle that has stayed beneath the skin and caused a flesh-colored bump to form. If the comedo comes to the surface of the skin and opens up, it is called a blackhead because of its dark appearance. This darkness is not dirt, but melanin, a skin pigment, that has been exposed to air.

When comedones become inflamed — i.e. they become red and tender — they are known as papules. Papules look like small red or pinkish bumps.

Pustules are what many people think of when they hear the word pimple. A more severe form of acne lesion, a pustule is a papule topped by pus composed of white blood cells, dead skin cells and bacteria. They often look like pink bumps with a white center.

Large, inflamed, solid lesions underneath the skin are known as nodules. They can be quite painful and often lead to scarring. Nodules and cysts—large, deep, inflamed sacs of pus—are among the most severe forms of acne lesions and often occur together.