in most cases, a pimple crops up when a pore becomes blocked with excess oil and dead skin cells, both of which feed the growth of bacteria and cause a small infection in the pore.According to the American Academy of Dermatology, there are actually a few different kinds of pimples. One common type is called a pustule. This is the kind with a red or pink inflamed border and a yellow or white center. As the name implies, pustules contain pus. This is the substance that comes out when you pop a pimple.