Here's another great video from ByteSizeScience from the ChemMatters series called "The Chemistry of Acne":
This video describes the physiology behind how pimples form. In short, excess sebum (or skin oil) clogs a pore and traps dead skin cells inside. Bacteria proliferate inside the pore to feed on the trapped skin cells. White blood cells migrate to the pore to fight the bacterial infection, and the mix of dead skin, white blood cells, and bacteria form the pus core of the pimple. The video also has a great explanation of the chemistry behind how benzoyl peroxide fights the acne-causing bacteria.
Image credit: NIH, via Wikimedia Commons.