If you mean what causes it: it's caused by bacteria under the skin, either entering a small opening in the skin or even in the bloodstream.
If you mean how it first appears: The area will get red and start to swell. It gets very painful and warm to the touch (that's the inflammation response to the infection). After a few days a whitehead forms.
If you have one, putting hot compresses on it can hurry up the process so it will heal quicker. Cold, however, can help reduce the pain.
at 1:06 AM on Jul. 13, 2010