Root canal treatment (also referred to as root canal therapy or endodontic therapy) is made necessary when a cavity is allowed, through neglect, to reach all the way to this pulp. (Regular cleanings and checkups prevent and detect problems early).
Sometimes deep restorations or trauma to a tooth may cause the nerve to be damaged to the point it needs root canal therapy, also. Once this occurs the pulp becomes infected, and can even extend through the root tip and begin to eat away at the surrounding bone (this is an abscess). By the time the pulp is infected it must be treated, and cannot heal on its own. It can even weaken the entire immune system. This is dangerous, not to mention very painful. Symptoms that the pulp has become infected may include sensitivity to hot/cold or sweets, pain, swelling, pain to biting or pressure, and a bad taste in the mouth.