Inlays and onlays are made to measure in the laboratory from either ceramic, gold or composite . Thy are cemented onto the tooth using a dental adhesive or composite resin.

They are used in cases where the cavity is too large to place a satisfactory direct composite restoration - their advantages over direct composites in such cases are the ability to  reproduce the exact occlusal form and anatomy of the tooth, and to create good contact points with neighbouring teeth. Their other advantages are greater strenghth  and  better aesthetics than direct composites.

Ceramic and composite inlays and onlays match the colour of the tooth , however gold inlays may be preferable if there is a heavy bite or bruxism. 

source : infosoins.fr et dentairemedia.info