What a root canal attempts to do is remove the diseased tissues (indicated by the yellow arrow), which contains the nerve and blood supply of a tooth, with a rubber type material. The area that is filled is called the "root canal." The root canal of each tooth is very different. The reason this tooth needed a root canal was due to decay (indicated by the white arrow) entering the pulp.

This is what the completed root canal looks like on an x-ray. This tooth had three canals, which you can see by the white filling material in the root portion of the tooth.