The  post itself,  is a metallic or reinforced fiber  structure that is placed within the  root of a tooth that has had a root canal (endodontic proceedure). When a root canal is  performed, the nerve is removed and the space it occupied is filled with a special filling material. 

In placing a  post, we remove some of this root canal filling material creating a space. The space that is created is used to anchor the post within the root of the tooth.

The type of post  that is chosen, will depend upon a number of factors. Most posts used today are prefabricated. The appropriate size is selected depending on the tooth.  

A core is built from filling material around the part of the post that sticks out of the root after the post is cemented into the root. The core is shaped so that it will anchor and retain a crown.  

Although posts are usually recommended when there is minimal support for a crown, they are not always necessary. The use of a post is determined on an individual basis based upon support and structural strength required.