Inlays and Onlays are lab-made restorations that are placed on teeth when the cavity or lost tooth structure is too large to be restored by a simple filling. The process of making an inlay is very similar to a crown. After the tooth is prepared, it is cemented or bonded to the tooth.
There are different materials that inlays and onlays are made of including gold, porcelain, and composite resins. Porcelain and composite inlays and onlays are more cosmetic as they are made to match your natural tooth color as much as possible. Gold is a suitable alternative, but its appearance makes it less popular.