A boil, also referred to as a skin
abscess, is a localized infection deep in the skin
. A boil generally starts as a reddened, tender area. Over time, the area becomes firm and hard. Eventually, the center of the abscess softens and becomes filled with pus. Finally, the pus "forms a head," which can be surgically opened or spontaneously drain out through the surface of the skin.
Boil on buttocks is usually due to plugged sweat glands that become infected or by an ingrown hair.