Inlay, can be porcelain, gold, or composite resin cast sections that are bonded to the damaged area of a tooth.  An inlay is similar to a filling and lies inside the cusp tips of the tooth; an onlay is a more extensive reconstruction that covers one or more cusps of a tooth. 

Inlays and onlays are both traditionally made of gold; however, porcelain is now becoming the material of choice because of its strength and potential to match the natural color of your tooth.

Inlays and onlays are applied in two dental visits.  At the first visit, the old filling, or decay, is removed, and the tooth is prepared for the inlay / onlay.  The dentist will then make an impression of the tooth, and send this impression to a dental laboratory.  This impression will be used by the laboratory to construct, a porcelain, or gold inlay / onlay that will fit exactly into your tooth.  At this time the dentist will place a temporary sealant on your tooth and schedule a second appointment.

At the second visit, the temporary sealant is removed.   Your dentist will then ensure that the inlay / onlay fits properly in the tooth and does not interfere with your bit.  Once correctly fit, the inlay / onlay is bonded into the tooth with a strong bonding resin, and polished smooth.

Considerations
Typically fillings can reduce the strength of the tooth by up to 50%. Inlays and onlays, however, because they are bonded directly into the tooth with special resins, can actually increase the strength of the tooth by up to 75%; as a result, they can last from 10-30 years, and in some cases, provide a good alternative to a crown.