Stage 1: The area around a hair follicle becomes blocked by dirt or another foreign substance such as grease.  Then, the sebaceous gland produces oil which is unable to make it to the surface and therefore causes the skin to rise.  This oil becomes infected with bacteria and causes the redness on the skin to worsen.

Stage 2: The blocked oil becomes even more infected and the pores expand more, causing redness to increase again and the skin to become drier.  Acne will not be fully developed yet, but the skin will share very small bumps that are a sure sign of a break out.

Stage 3: The pores clogged with bacteria infected oil become too full and either explode onto the skin or implode into other pores.  This causes full blown acne to develop and even start to spread to other areas of the skin.

Stage 4: Your body realizes there is an infection where the acne is and sends antibodies and blood.  This actually will make things worse before they get any better because all the extra blood and antibodies in the area will make your skin red.  This might never cure your acne depending on how severe it is, your genetics, and external conditions.