Patient-friendly definition from the DentistsDirectory.org dental glossary.
The term "root canal" has two distinct meanings in dentistry that are often confused. Anatomically, a root canal is the natural hollow channel that runs through each root of a tooth from the pulp chamber in the crown down to the tip of the root. Every tooth has at least one root canal — front teeth typically have one; premolars may have one or two; molars usually have three or four. These canals house the dental pulp, which contains the tooth's nerve and blood supply.
As a procedure, "root canal" (correctly, root canal treatment or endodontic therapy) refers to the process of removing the infected or damaged pulp from these canals, thoroughly cleaning and disinfecting their walls, shaping them to receive a filling, and sealing them permanently with a rubber-like material called gutta-percha. The purpose is to eliminate the bacterial infection that has invaded the pulp and prevent reinfection. Root canal treatment saves teeth that would otherwise require extraction due to infection, preserving natural tooth function and preventing the bone loss and shifting that follows extraction.
The enduring reputation of root canal treatment as painful is largely outdated and undeserved — it comes from an era of inadequate anesthesia. Modern root canal treatment with effective local anesthesia is no more uncomfortable than a routine filling for most patients. What is painful is the dental infection before the procedure; the treatment relieves that pain. Most patients are surprised by how manageable it is. Treatment on a front tooth is usually completed in one appointment; molar root canals typically take one to two appointments. A crown is usually recommended afterward to protect the now more brittle tooth from fracture.