Inlays and onlays are restorations that are made in the dental laboratory and require two visits to complete. These are an upgrade from fillings and are used when regular fillings are not adequate to restore the teeth to function. That means when the side of a tooth is broken or the hole is too large for just a filling an inlay or onlay is used.
The difference between inlays and onlays is that an inlay usually fills a hole created by the dentist and restores one or two sides of the tooth. An onlay is usually larger and restores the sides of the tooth as well as part or all of the chewing surface.
Inlays and onlays can be made of different materials including gold, resin composite or porcelain. As with fillings there is a big difference in longevity among these three materials with gold of course being the longest lasting by far. Choice of material and restoration is guided by size, location, function, and esthetics.