Acne is the most common skin condition in the United States. Acne includes: Blackheads, Whiteheads, Papules, Pustules (what many people call pimples), Cysts, Nodules. It appears all over the body including the face, back, buttocks and neck.
From the American Academy of Dermatology:
Acne appears when a pore in our skin clogs. This clog begins with dead skin cells. Normally, dead skin cells rise to the surface of the pore, and the body sheds the cells. When the body starts to make lots of sebum (see-bum), oil that keeps our skin from drying out, the dead skin cells can stick together inside the pore. Instead of rising to the surface, the cells become trapped inside the pore.
Sometimes bacteria that live on our skin, P. acnes, also get inside the clogged pore. Inside the pore, the bacteria have a perfect environment for multiplying very quickly. With loads of bacteria inside, the pore becomes inflamed (red and swollen). If the inflammation goes deep into the skin, an acne cyst or nodule appears.