Root canal therapy (RCT) is the only alternative to extraction for teeth with injured or infected pulp (nerve) tissue. During the procedure diseased or dead tissue from the central part of the tooth will be extracted, and the void sealed with filler. A common misconception is that RCT is painful. The truth is, RCT doesn't cause pain, it relieves pain. The perception of root canals being painful began decades ago before advent of modern technologies and anesthetics. Today, RCT is no more uncomfortable than having a filling placed. In fact, a recent survey showed that patients who have experienced RCT are six times more likely to describe it as "painless" than patients who have not had it.
The detailed explanation of the procedure can be found by clicking onto this link
After the procedure, temporary restoration is put in place to seal the tooth and protect it from re-infection by bacteria from the mouth. At this point, a significant part of tooth structure is missing because access to the root canals during the procedure required making an opening directly above the canals, and in addition extensive damage due to tooth decay or trauma usually has been present (and was the reason why RCT was done). Therefore, the missing part of the tooth has to be restored with permanent filling (core). Then, with rare exceptions, a crown has to be placed to ensure longevity of the tooth by preventing its fracture and re-infection of the root canals.