Infected cyst. A boil is an abscessed structure under the skin.
Skin abscess. A boil is a localized infection in the skin that generally starts as a reddened, tender area. Over time, the area becomes firm, hard, and tender. Eventually, the center of the boil softens and becomes filled with infection-fighting white blood cells from the bloodstream to eradicate the infection. This collection of white blood cells, bacteria, and proteins is known as pus. Finally, the pus "forms.