A boil is a localized infection in the skin that begins as a reddened, tender area. Over time, the area becomes firm, hard, and tenderer. 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 a head,"" which can be surgically opened or spontaneously drain out through the surface of the skin. Pus enclosed within tissue is referred to as an abscess.

A boil is also referred to as a skin abscess.

There are many causes of boils. Some boils can be caused by an ingrown hair. Others can form as the result of a splinter or other foreign material that has become lodged in the skin. 

Others boils, such as those of acne, are caused by plugged sweat glands that become infected.