Picture of Boil
A boil is a skin abscess, or infection, that occurs deep within the skin. A boil may start as a tender, red bump. As it grows, it will become hard and firm. The center will fill with white blood cells, proteins, and bacteria (pus) to fight the infection. This white center moves closer to the surface of the skin at which point a doctor can lance the growth to drain it. Sometimes boils open and drain without assistance.
Text Reference: Reference: American Osteopathic College of Dermatology: "Boils"