Q: What's the difference between a whitehead and a blackhead?
A: Your skin is covered in tiny sebaceous glands, which secrete sebum, a substance which moisturises your skin. When these get clogged with plugs of sebum and keratin, you break out and get whiteheads and blackheads, which are a type of acne that is not inflamed.
A blackhead (open comedo) is a wide plug near the surface of the skin. Because it's exposed to air, it becomes oxidised and turns a dark colour. The dark colour isn't dirt!
A whitehead (closed comedo) is made up of the same sebum and keratin, but there is only a small opening to the surface. Since it's protected from air, the plug doesn't oxidise, and stays a white colour. The difference between a proper whitehead and a pimple is that a pimple is inflamed from bacterial infection.