You can think of a dental inlay or onlay as being midway between a filling and a crown. Inlays or onlays are used when not enough tooth structure remains to support a filling, but the tooth is not so severely damaged that it needs a crown.
An inlay is similar to a filling, but it lies within the cusps (bumps) on the chewing surface of your tooth. An onlay is more extensive than an inlay and covers one or more cusps.
Inlays or onlays can be made of gold, composite resin (plastic) or ceramics. They can last for decades. However, how long they last depends on the material used, the teeth involved, the forces of chewing and how well the patient maintains them with good oral hygiene and regular visits to a dentist.