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. That doesn't happen often, though.
