Sebum, which comes from the Latin word for "fat," is an odorless oil produced by the sebaceous glands. The substance contributes to oily hair and skin, while overproduction of sebum can cause acne. As sebum breaks down, the bacteria gives off an unpleasant smell, which can contribute to body odor.
Sebum isn't all bad, though: It's also what coats your hair to make it nice and shiny. In addition, sebum protects skin from infection and keeps it from dehydrating. Sebum production slows down with age, so post-menopausal women produce less sebum.
One common misconception is that powder and blotting papers can reduce sebum production. Though these products will remove oil on the surface of your skin, they will not decrease the amount of sebum your body makes.