Boils (furuncles) are painful, pus-filled bumps that form under the skin when bacteria infect and inflame 1 or more hair follicles.
Boils start as red, tender lumps. These fill with pus, grow, then rupture and drain. A carbuncle is a cluster of boils.
Warm compresses treat most boils. Large boils and carbuncles need a doctor's attention.