The type of acne that a lot of teens get is called acne vulgaris (the meaning of "vulgaris" isn't as bad as it sounds — it means "of the common type"). It usually shows up on the face, neck, shoulders, upper back, and chest.
The hair follicles, or pores, in your skin contain sebaceous glands(also called oil glands). These glands make sebum, which is an oil that lubricates your hair and skin. Most of the time, the sebaceous glands make the right amount of sebum. As the body begins to mature and develop, though, hormones stimulate the sebaceous glands to make more sebum.
Pores become clogged if there is too much sebum and too many dead skin cells. Bacteria (especially one called Propionibacterium acnes) can then get trapped inside the pores and multiply. This causes swelling and redness — the start of acne.
If a pore gets clogged up and closes but bulges out from the skin, you're left with a whitehead. If a pore gets clogged up but stays open, the top surface can darken and you're left with a blackhead. Sometimes the wall of the pore opens, allowing sebum, bacteria, and dead skin cells to make their way under the skin — and you're left with a small, red bump called a pimple (sometimes pimples have a pus-filled top from the body's reaction to the bacterial infection).
Clogged pores that open up very deep in the skin can cause nodules, which are infected lumps or cysts that are bigger than pimples and can be painful. Occasionally, large cysts that seem like acne may be boils caused by a staph infection.
Below is a video on Rhonda Allison's 90 Day Acne Sessions