Whiteheads are pimples that are filled w/ pus and this is what gives them their name.  Like most acne, whiteheads are formed when excess oil from the sebaceous glands clogs up the pores or hair follicles on the skin.  This sebum gets trapped along with other dust and some bacteria w/c can be airborne or from the skin's surface so that when the trapped sebum and bacteria stay below the skin's surface, a whitehead is formed. Whiteheads can show up as tiny white spots, or they may be so small that they are invisible to the naked eye.  As whiteheads are involved off from the outside air, they remain white.  Whiteheads are often found hidden away such as on the forehead under long bangs.  If such whiteheads were to open sufficiently to allow the outside air in, they would turn into blackheads.