The formation of bumps is (among other mechanisms) a result of pores getting clogged with oil and dead skin cells. Oily skin typically appears as a shiny and thick surface on the skin as a result of overactive sebaceous glands. The sebaceous glands located at the hair follicles produce sebum, which is a waxy, oily material naturally moisturising our skin. Furthermore, the pores often seem enlarged. People with oily skin often get a feeling of having greasy skin even after a couple of hours after cleansing.