A crown is a 'cover or cap' made of gold, zirconium or porcelain that is placed over the tooth to restore it permanently. An onlay is a filling made of one of these durable materials and a veneer is a shield made of porcelain on the front of the tooth.
Crowns or onlays are needed, for example, when a filling has become very large (functional) or your own tooth is very damaged or discoloured and you want it to look nice again (aesthetic). In the latter case, you can also opt for a veneer because you then need to grind less of your own tooth.
The crown has the advantage over a filling that the connection is better on a microscopic level and therefore there is less risk of new cavities in the long run, and the material of crowns wears or discolours much less than the filling material.
When your own root is no longer healthy or is already absent, a crown can be placed on an implant. This is an artificial root made of metal, usually titanium.