Inlays are fillings bonded to the inner part of the tooth whereas onlays cover the biting surface. They can serve as an alternative to a large filling, which can weaken the structure of the tooth and require a crown, which caps the tooth.
They increase the strength of the tooth and can last for decades. Inlays and onlays can take a few appointments to remove old fillings, if necessary, to take impressions in order to make custom inlays and / or onlays, and then a temporary filling is placed until your next appointment when the inlays and onlays are put in place.