Whiteheads and blackheads refer to closed and open comedomes, which are a form of acne. To be more precise, there are two categories of acne: pimples and cysts in one group, and whiteheads and blackheads in the other.
Comedomes are not as blatant symptoms of acne, as pimples or cysts, but most certainly should be addressed. Usually they are more common and numerous on the face and shoulders, or back.
Acne appears as a result of an inflammation caused by the excessive sebum (skin oil) which clogs the skin pores.
Blackheads, or open comedomes, are actually hair follicles that are opened wider than regular folicles. They are filled with plugs of sebum and dead skin cells deposits, and undergo a chemical process which results in an oxidation of the melanin. This reaction gives the typical black color of the substance in the hair folicle.
Whiteheads, or closed comedomes, are hair folicles closed with the sebum, but unlike blackheads, they have only a microscopic opening to the skin surface. The air can not reach to the substance in the folicle, so it does not oxidize, and remains white.