Acne is a skin condition that occurs when you get a build up of dead skin cells and sebum oil that gets trapped under the skin inside the pores. It can create a bump on the outside or inside depending on the type of pimple it creates. There are a few different types of acne measured by severity such as mild acne which includes pustules, black heads and whiteheads. Cystic and nodule acne are more severe which can cause painful lumps under the skin. These can also leave behind red marks on the surface of the skin.