These are restorations that are best described as laboratory constructed fillings. They are made of either gold or ceramic. They are cemented into the tooth and form a very strong bond with the tooth surface.
They are used in larger cavities or when a tooth is heavily broken down. They have been largely superseded by modern composite materials and thus not used as often as they used to be. Often, one sees 30 year old Gold inlays looking as good as they did the day they were originally fitted!