An Inlay is an indirect restoration consisting of a solid structure (precious metal, porcelain cured composite resin) which is made in a laboratory on models constructed from impressions taken in the mouth after the tooth has been prepared by your dentist. Once made, they are held in place by dental cement. They are generally stronger than fillings placed in a soft state in the teeth which then set.
An Onlay is very similar to an inlay but is generally a larger filling replacing at least one cusp, the biting surface generally covers more of the tooth to protect it from the pressure of chewing.
Inlays and onlays are not frequently used restorations as modern filling materials placed in a soft state have become much stronger, they are used when patients with a heavy bite continually break standard fillings.
As an NHS restoration they fit into the Band 3 charge structure.