An abscessed tooth is a pocket of pus, usually caused by some kind of infection and the spread of bacteria from the root of the tooth to the tissue just below or near the tooth.
In general, a tooth that has become abscessed is one whose underlying pulp (the tooth's soft core) has become infected or swollen. The pulp contains nerves, blood vessels and connective tissue, and lies within the tooth. It extends from the crown of the tooth, to the tip of the root, in the bone of the jaws.
An abscessed tooth can be an extremely painful condition.
In some cases, antibiotics are administered in an attempt to kill an infection. If antibiotics are ineffective and an abscess is shown to be damaging the pulp or lower bony structures, a root canal procedure may be needed to remove the dead pulp and restore the tooth to a healthy state.