Inlays/onlays are solid, one piece restorations fabricated indirectly and then cemented into place. An inlay will fits into a cavity within a tooth as a filling does, while an onlay will extend further outside the cavity to cover over a cusp of the tooth.
Conventional direct restorations like amalgam and composite resins are subjected to setting expansion and shrinkage respectively, thus exerting great pressure on the tooth structure. These undesired setting pressures may result in fractures, loss of marginal integrity, leakage and potential decay and tooth loss. Inlays/onlays do not shrink or expand upon cementation, and thus do not have the undesired effects on the tooth. Fabrication of these restorations indirectly ensures that their fit would be far superior, more aesthetic, and more durable than your conventional fillings.