Inlays and onlays are similar to fillings in the sense they are used to restore a tooth following damage but the main difference is they are custom made and cemented in to place.
They are often used when there is not enough tooth left to place a regular filling.
In these circumstances, an inlay or onlay will give a much stronger result.
Your dentist will advise on the advantages/suitability of inlays and onlays and what combination is best in your individual circumstances.