Acne is caused by androgen hormones which cause sebaceous glands to grow and produce more sebum (an oily substance). Sebaceous glands are normally ocuring and connect to the hair follicles.

The additional sebum and the normally created dead cells clog the hair follicles. This condition attracts bacteria living in the skin to attack the hair follicles, causing inflammation and forming acne.

What happens is that after the bacteria colony starts, a whitehead or a black head forms on the hair follicle. The whiteheads and the blackheads should not be squeezed. Squeezing increases the problem rather than resolving it. 

If the site becomes infected, puss appears, forming a yellow top. At the point that you get infections you need to see a doctor as acne infections can cause scaring. Some people become extensively scarred and must receive special treatment.