Root refers to the component of an individual tooth that is buried in the gum and connects the tooth to the jaw. Within the interior space of the tooth root lie the root canals and the pulp. There are at least one and as much as four root canals within a tooth that lie at the tip of the tooth's root. The root canals run through the center of the root to the tooth's pulp chamber which contains the tooth's pulp. The pulp refers to the softest part of the tooth that lies within its root and extends all of the way to the top part of the tooth (the crown). It contains the blood vessels nerves and connective tissue within a tooth and provides the tooth's blood and nutrients. Essentially the pulp is the lifeline to the tooth and there is a branch of dentistry endodontics which specializes in its treatment.
Endodontic treatment may be required if abscess (a pocket of pus) forms at the tip of the tooth's root as this is a sign that there is damage to the tooth's pulp. With root canal treatment the intention is to remove the infected pulp of a tooth while retaining the tooth in order to avoid extraction.
However in instances where root canal therapy is insufficient and tooth extraction is required the tooth's root can remain. These roots are known as retained roots and have been known to cause infection of the jaw or osteitis. However retained roots are also used as a way to provide support to overdentures which are full or partial dentures designed to fit over retained teeth or roots.