Picture of Pilonidal Cyst
A pilonidal cyst is a unique kind of abscess that occurs in or above the crease of the buttocks. Pilonidal cysts often begin as tiny areas of infection in the base of the area of skin from which hair grows (the hair follicle). With irritation from direct pressure, over time the inflamed area enlarges to become a firm, painful, tender nodule making it difficult to sit without discomfort. These frequently form after long trips that involve prolonged sitting.
Boils Slideshow: Watch & Learn About Boils
Image Source: MedicineNet
Text: "Boils", MedicineNet