Folliculitis is inflammation, and generally superficial infection of the hair follicles. It is characterized by red bumps and pustules. It may be caused by bacteria (most common staph aureus), fungi, viruses, or parasites.
Furuncle (a boil) is an infection under the skin that causes a painful, pus-filled nodule. It involves the hair follicles and usually occurs in high-friction areas such as the neck, underarms, groin, buttocks, and between the thighs.
Carbuncle is several inflamed follicles into a single inflammatory mass.
Abscess is a collection of pus in the deeper skin tissues. It is typically painful, tender, and red. Generally these require incision and drainage of the abscess and antibiotics for treatment.
Cellulitis is an infection of the skin with or without abscess. It occurs when bacterial enters the skin when the skin barrier is compromised (such as trauma, skin inflammation, swelling, and obesity). It results in redness, warmth, and tenderness.