Root canal therapy
Note that with new dental technology, this procedure is totally pain free and most of times will be completed in one appointment.
Root canal therapy is a dental treatment to save a severely decayed or infected tooth. Each tooth has one chamber, one, two or three roots and multiple canals inside the roots. Pulp is the nerves and blood vessels that exist with the chamber and the canals. When pulp is infected and damaged, bacteria begin to multiply within the pulp chamber causing infection, abscessed tooth, and bone loss.  The symptoms can include severe toothache, cold and hot sensitivity, swelling of gum, pimple on the gum or sometime even no symptoms at all.
The Root Canal Procedure
First, the tooth and the area are completely numbed. Then an access hole is drilled into to the tooth.
Note that this procedure requires multiple x-rays. During the treatment, the infected pulp is removed. The cleaning-out process is accomplished using root canal instruments and dental disinfectants. Once the tooth is thoroughly cleaned, the canals are filled and sealed.   
The final step involves further restoration of the tooth. This step will help avoid recontaminating the tooth’s interior and also may prevent a fragile tooth from breaking before the tooth can be fully restored. In this procedure, the tooth is restored with a post core built and a crown.
For the first few days following the completion of treatment, the tooth may feel sensitive due to natural tissue inflammation, especially if there was pain or infection before the procedure. You may call your dentist for a consult.