Acne occurs when hair follicles become blocked with dead skin cells and oil. These follicles are connected with sebaceous glands which secrete an oily substance. This oily substance is known as Sebum, which lubricates the skin and hair.
In normal conditions, sebum travels up along the hair shafts and then moves out, through the openings of the follicles to the skin surface. However, when sebum and dead skin cells are produced in excessive amount, the two get coagulate in the follicles and form a plug. It creates a favorable environment for the growth of bacteria. Later, this plug can cause whiteheads (small bulging white bumps) or blackheads (oxidized plug).
Pimples appear on the skin as red spots, with a white center. The white area indicates inflammation or infection, which is caused due to the blockage of hair follicles.
Cysts are a result of inflammation or infection which is developed deep inside the follicles. However, keep in mind that the openings of the sweat glands do not develop acne.
Some studies have proven that acne can be genetic. Besides, medications which contain androgen and lithium, cosmetics containing harmful chemicals and hormonal changes in the body are some other factors that are blamed to cause acne.