Inlays are similar to fillings but the entire work lies within the cusps (bumps) on the chewing surface of the tooth. Onlays are more extensive than inlays, covering one or more cusps. Onlays are sometimes called partial crowns. Overlays extends outwards from onlays.
Inlays and onlays are normally done to repair damaged portion of teeth that may be too large to be fixed with composite resin fillings whilst the damage is not overly extensive requiring a dental crown.
Inlays and onlays can be made of porcelain, gold.