While a cyst is a smooth, round, closed sac under your skin filled with fluid or semisolid material. A carbuncle is a few boils grouped together. A boil is a small red pimple-like thing with symptoms like redness, swelling, pain, a white or yellow center, oozing pus or crusting, fatigue, or fever. It’s formed by bacteria in the skin, and the most common places where it begins include the neck, breasts, face, armpits, buttocks, and thighs.