For reasons no one completely understands, follicles, often called pores, sometimes get blocked. Sebum (oil) which normally drains to the surface gets blocked and bacteria begins to grow. Both whiteheads and blackheads start out as a microcomedone.
Microcomedones become non-inflamed skin blemishes called comedones--either a whitehead or a blackhead:


CM-Beauty solutions to keep acne concealed: 
 
Face Magic

Leg & Body Magic

Coverstik