Oily skin is the outcome of an overproduction of sebum from your sebaceous glands, located under the skin's surface.
Sebum is an oily substance made of fats and its main function is to lubricate, protect, and hydrate the skin to prevent dryness and dehydration. In short, all skin types produce sebum. The problem with Oily Skin types is an overproduction of sebum.
Too much sebum can make skin look shiny or greasy throughout the day. The overproduction of sebum can also cause breakouts or congestion, as the oil can get trapped with all the dead skin cells and impurities in the pores.
However, don't curse your oily skin just yet! There are also good news that come with this skin type. The benefit of having oily skin, is you will actually have fewer lines and wrinkles than a drier skin type!