A "veneer" is a wafer-thin layer of material molded to the surface of a tooth to correct a chip or crack, or to enhance its cosmetic appearance. Veneers are made of either porcelain, or a composite synthetic resin, such as acrylic polymer or polymethyl methacrylate. These synthetic resins are liquid materials that can be converted into a permanent hardened material, and are used in dentistry because of their ability to create a strong bond with the tooth, and their ability to mimic the natural color of adjacent teeth. Lastly, a veneer can either be layered directly onto a tooth at the dentist office, or fabricated off-site in a dental laboratory.
An "onlay" is a form of dental restoration used to repair a decayed, chipped, cracked, or otherwise damaged tooth. An onlay corrects damage extending to a "cusp" (the raised points on the biting surface), in contrast to an inlay, which is used to repair only that portion of the tooth between these cusps.
Generally speaking, there are two different types of dental restorations: "direct" and "indirect." For example, a "filling" is a type of direct restoration, because the material used to repair the tooth undergoes its hardening process while in "direct" contact with the tooth, and *inside* the mouth.