Fillings are made of composite resin and bonded directly onto the prepared tooth while inlays and onlays are made of milled porcelain and custom-made using impressions of your mouth that are provided to a dental lab to design. Fillings and inlays restore chewing surfaces while onlays restore both the chewing surfaces and cusps of a tooth. Inlays and onlays are more expensive but can last longer than fillings.