We can all agree that pimples are a real nuisance. They can range from being a slight eyesore on an otherwise perfectly clear complexion, to being huge and angry monsters that cause you overwhelming discomfort and distress. So what causes pimples to occur? Sebaceous glands are glands in our skin that secrete an oily substance called sebum into hair follicles that lubricates hair and skin to protect them. When these glands become too active, dead skin cells will clog up the pores, resulting in the formation of pimples, with infections happening in some cases. You will usually find pimples popping up on the face, chest, shoulders and backs because these are the areas with a large concentration of sebaceous glands. Here are the types of pimples listed from the least severe to the most severe, and what you can do to treat them effectively.