WebMD explains that most pimples are caused by an excess build up of sebum that gets trapped in the pores. Bacteria grows and a pimple forms at the sight. One cause of pimples is hormones. An imbalance in the body's hormones can cause cystic and nodular acne to erupt.
White heads form due to the body's white blood cells fighting off the bacterial growth. Blackheads are formed from dirt and other things clogging the pores. The source of a pimple is determined by the type of pimple that erupts. Various forms of pimples are: comedones, blackheads, whiteheads, papules, cystic, nodule, pustule or hormonal skin imperfections.