Root canal therapy or endodontics refers to the process by which a dentist treats the inner aspects of a tooth, specifically that area inside a tooth that is occupied by its pulp tissue. Most people would probably refer to a tooth's pulp tissue as its nerve.
Initially a tooth's pulp (nerve) tissue plays an important role in the formation and development of the tooth. Once the tooth has formed, this tissue helps preserve the tooth's health and vitality.
The purpose of root canal treatment is to make sure that the tissues surrounding a tooth's root will remain healthy despite the fact that the pulp has undergone degenerative changes and has been removed. With competent root canal therapy which removes a diseased pulp, the tissues surrounding a tooth's root heal and remain healthy.
The process of root canal treatment first removes (as thoroughly as possible) bacteria, pulp tissue, the organic breakdown of pulp tissue and bacterial toxins from within the inner aspects of a tooth. Left untreated, these byproducts can produce tissue irritants that can cause inflammation.
Once the inside of the tooth has been cleaned, the second half of root canal treatment involves filling in and sealing up the interior of the tooth, much like placing an internal bandage. This aspect of the treatment is an attempt to prevent bacteria from forming within the inner aspects of the tooth.