Inlays are used instead of fillings to replace small amount of tooth structure loss due to tooth decay. Like fillings, inlays fit inside the tooth and can be made up of a tooth-colored material such as ceramic/porcelain, special dental composite or gold.
Onlays also fit inside the tooth but extend to the chew surface of back teeth to replace one or more cusp. Like inlays, a variety of materials can be used to make onlays.
For both inlays and onlays, bonding the material to the tooth may actually improve the strength of the tooth and help seal the material to the tooth.