According to WebMD, cystic acne is a form of skin blemish that occurs when the infection beneath a pimple finds its way deeper underneath the skin. This causes an inflamed, reddened blemish that is more painful than a typical acne blemish.
Wikipedia further describes cystic acne as a severe, inflammatory case of acne. Cystic acne is common on body parts where sweat collects, such as around the buttocks, groin, armpits and other areas heavy with perspiration ducts. Cystic acne can result in physical scarring and psychological distress, and it often requires the attention of a medical professional for effective treatment.