A prosthesis is the replacement of a missing part. A dental prosthesis replaces missing teeth.
When the root and most of the structure of a tooth is intact, the tooth can be restored with a crown.
When the root is intact, but there is insufficient tooth structure, a post core is inserted into an endodontically treated tooth (ie., one that has had root canal treatment), and then the crown is placed on top of that.
When the entire tooth is missing, including the root, a fake tooth has to be either supported by adjacent healthy teeth, or on an implant which replaces the root portion. Implants can be used to replace individual teeth which sit upon them, or as terminal supports to replace missing teeth in between them.
When fake teeth are held in place by crowns covering natural teeth or implants on either side of them the entire prosthesis is called a fixed bridge
When fake teeth are held in place by clasps hooked around natural teeth or crowns on either side of them the entire prosthesis is called a removable bridge or partial denture
When there are no teeth and they are all replaced by a prosthesis, it is called a complete denture.
When a complete denture is given additional retention by snapping onto implants or root stumps which have been modified with special attachments, the prosthesis is called an overdenture.