There are times when a tooth suffers damage (from decay, for example) that is too extensive to be treated with a simple filling — but not extensive enough to need a full-coverage crown. In these cases, the best option for restoring the tooth may be an inlay or onlay.
Both inlays and onlays are considered “indirect” fillings, meaning that they are fabricated outside the mouth (generally at a dental laboratory), and then bonded to the tooth by the dentist.