Each of the hair follicles on your body is connected to glands that secrete an oily substance to lubricate your skin and hair. To do that the substance, known as sebum, needs to move up the hair shaft and out the opening of the follicle. But sometimes the sebum acts almost like a glue to dead cells being shed by the skin. Together, they form a soft plug that blocks the shaft, while the sebaceous glands continue to produce sebum. The growing blockage can stimulate local bacteria like staphylococcus to reproduce. Increasing bacteria leads to a response by the immune system. A pimple is the visible evidence of this process--white pus formed from sebum and dead cells and bacteria, and redness from inflammation as the immune system tries to kill the bacteria and heal the tissue.