A tooth has pulp which is covered by two protective layers called enamel & dentin, but which can be decayed, which we call as Caries / Cavity.
A tooth consists of three layers. The innermost is Pulp covered by Enamel & Dentin.
Decay in enamel or upper part of dentin can be treated by filling (Composite). If neglected, the decay proceeds towards the pulp, leading to infection of pulp & thus, can be treated by root canal.
A badly decayed tooth, apart from root canal, may also require additional support. In such a case, a post (viz. pillar) is used around which core-buildup (refilling) is done to provide more strength to the tooth.
According to the tooth’s condition, a root canal can be done in a single sitting or may require 2-3 sittings.