What is the difference between a whitehead and a blackhead?

The ducts sebaceous glands usually empty into hair follicles, but some open directly onto the surface of the skin. If the duct of a sebaceous gland becomes blocked by sebum, a whitehead appears on the skin surface. If the accumulated material oxidizes and dries, it darkens, forming a blackhead. If the sebaceous glands become infected, resulting in pimples on the skin, the condition is referred to as acne.