Your skin is pierced with innumerable pores, tiny holes that lead to small sacs - the sebaceous follicles. The latter each contain a hair and a gland that produces sebum (sebaceous gland). When the cells lining the walls of a pore pile up on its opening and clog it, the sebum can no longer flow: it accumulates within and you end up with a "whitehead", which is also referred to as a "closed comedone" or microcyst. Sometimes the microcysts disappear by themselves. But most often, they continue to evolve: the sebum accumulates more and more, until it breaks through the crust of dead cells. It then takes on a black colour on contact with air: the whitehead has become a blackhead.