How do i know exactly what is an abscess or abscessed tooth?

Your worst nightmare: Pus enclosed in the tissues of the jaw bone at the apex of an infected tooth's root. When decay damages the tooth into the nerve, it creates a restriction of oxygenated blood into the pulp tissue. This in turn creates a pressure build up in the area outside the tip of the root where the nerve exits. Like a garden hose with a kink, this knot will create an immense pressure build up and pain!

Infection: It usually comes from a bad or dying tooth. The infection is trying to get out of the tooth and out of your body, so it will create a swollen area of pus, just like a pimple.