Acne is formed by the blocking of the skin duct surrounding hair follicles. Sebum and dead skin cells combine to block the skin duct resulting in the formation of various types of acne blemishes.
Normal Follicle:
Microcomedo:
This is the beginning of a blemish. A microcomedo is formed when sebum and dead skin cells block the pore opening. At this stage the blockage is invisible to the eye and may take as long as eight to ten weeks to develop into a visible blemish.
Blackhead:
As the plug becomes larger and more fully blocks the pore opening it changes from a microcomedo to a comedone. If the comedone is open to the air, it oxidizes and becomes dark in color forming what we know as a blackhead.
Whitehead:
If the comedone forms beneath the surface of the skin and is closed to the outside air it does not discolor but stays white. This is what we know as a whitehead. It often contains debris that is deep and impacted and can lead to the formation of a papule or pustule.
Papule:
P.acnes bacteria, normally found on the skin, combines with sebum and result in inflammation and redness. At this point the inflammation is under the surface of the skin and the result is a raised, red, solid blemish.
Pustule:
Additional inflammation and pus may form causing a red, raised blemish that is often painful to the touch. Pus is a mixture of white blood cells, dead skin cells and bacteria.
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This is the beginning of a blemish. A microcomedo is formed when sebum and dead skin cells block the pore opening. At this stage the blockage is invisible to the eye and may take as long as eight to ten weeks to develop into a visible blemish. 
As the plug becomes larger and more fully blocks the pore opening it changes from a microcomedo to a comedone. If the comedone is open to the air, it oxidizes and becomes dark in color forming what we know as a blackhead. 
If the comedone forms beneath the surface of the skin and is closed to the outside air it does not discolor but stays white. This is what we know as a whitehead. It often contains debris that is deep and impacted and can lead to the formation of a papule or pustule. 
P.acnes bacteria, normally found on the skin, combines with sebum and result in inflammation and redness. At this point the inflammation is under the surface of the skin and the result is a raised, red, solid blemish. 
Additional inflammation and pus may form causing a red, raised blemish that is often painful to the touch. Pus is a mixture of white blood cells, dead skin cells and bacteria.