Acne is a skin condition that is caused by hair follicles in the skin becoming blocked. Hair follicles are tiny holes in the skin which produce small hairs, which are attached to sebaceous glands. These small glands lubricate the hair as it grows, and ensures that the skin doesn't dry out. To do this, they must produce a natural oil that is called "sebum."
People with acne will have glands that produce too much sebum, creating an oily appearance on the skin. As the oil mixes with dead skin cells that we all naturally have on our skin, it clogs up the pores and causes them to expand. This plug stops the sebum from escaping, and the pore will often continue growing until it burst. A blocked follicle that is close to the surface of the skin will create a pimple or a whitehead. A blocked pore that is deeper in the skin, however, will form a blackhead.
Acne sufferers may also experience infections that are caused by the usually harmless bacteria found on the skin. This creates cysts and nodules.