The word "comedo" comes from the Latin "comedere", meaning "to eat up", and was historically used to describe parasitic worms; transferred in medical use to secretions that resemble them.

It is a thickened secretion plugging a duct of the skin, especially of a sebaceous gland; blackhead.

A comedo typically occurs on the forehead and chin. If it is open, it is called a blackhead (open comedo), and if it is closed, it is a whitehead (closed comedo). The black isn’t actually dirt. It’s a mixture of keratin and sebum that darkens as it oxidizes.

Comedones are different from acne in that the surrounding tissue doesn’t become inflamed or contain pus or bacteria.

More Info: en.wikipedia.org