Root canal treatment, also called root canal therapy is the removal of damaged or infected nerves and tissue from the inside of your tooth. Inside the root of every tooth is a nerve, with pulp tissue surrounding and protecting the nerve and a tiny blood vessel that extends out the end of the root to your circulatory system. These spaces inside the roots are called the canal space or root canals.
The root canal procedure involves drilling a small opening in the tooth, removing the nerve, blood supply and pulp tissue. The canals are then cleaned, shaped with a special drill and sterilized. The end of the root and canal space is then sealed off with a filling material, much like a cavity or hole in your tooth would be filled. A root canal is a repair to the inside of the tooth and is the only cure for internal tooth nerve damage or tooth abscess besides having the tooth removed. A dental root canal allows you to keep a tooth you would otherwise have to have pulled. When there is infection inside the tooth, there is no cure. The bacteria continue to spread to the nerve and surrounding tissue most times causing an abscess in your jaw bone. Many people wonder why medications, like antibiodics, cannot cure the infection. The fact is that the tissues swell, cutting off the blood supply so there is no way to get the medication to the area. Yes, they can help surrounding tissue, but cannot reach the inside of the tooth.
Without proper treatment, a root canal abscess can become a life threatening situation. With your brain only inches away from your tooth, there have been cases of systemic infection that resulted in death.
Root canal treatment is one of life's most dreaded procedures. Al reputation has not been forgotten though teeth root canal done today are pain free, their bad reputation has not been forgotten.

This molor tooth and the bicuspid tooth beside it have been prepared for crowns or caps. You can tell by the squared look at the top. The white lines in the teeth show the root canal filling. This is gutta percha, a natural rubber.  According to Wikipedia, "Its physical and chemical properties, including but not limited to its inertness and biocompatibility, melting point, and ductility afford it an important role in the field of endodontics for a long time to come."