A dental “inlay” is a dental restoration that is often used to repair areas of decay that are too large to support a filling but not so large that a crown is necessary. Inlays generally cover chewing surfaces between cusps in molars, and “onlays” are used to restore fractured cusps. To prepare an inlay, the dentist makes a wax mold of the space left after the damaged portion of the tooth is removed. Then, the mold is sent to a lab, where the custom inlay is created (usually out of gold alloy). Finally, the inlay is set into place, using cement. The resulting restoration is more durable than amalgam or composite fillings but less expensive than a crown.
P.S. Porcelain is fast becoming the material of choice for dental inlays due to its strength and color-matching ability.