Inlays and Onlays are indirect fillings made by a dental lab. Instead of getting a composite resin filling or an amalgam filling, which can be delivered on the same day at the dental chair-setting, an impression is taken once all the cavities are removed from the tooth and prepped in such a way that the indirect filling can be inserted and cemented. The indirect fillings are made in different materials such as porcelain, zirconia, gold, etc. The price of indirect fillings are almost the same as crowns, but for most cases, it may not be worth the price. During taking the impression, human error can contribute to the failure of the indirect filling. Also, small human error from the dental lab can contribute to the failure such as open margin. Therefore, unless it is absolutely indicated, it may not be recommended.
Inlays are onlays are very similar but if the prepped area (missing area) involves a cusp, it is called onlays.