Composite Veneer:
It is directly placed (built-up in the mouth) by dentist in clinical setting and polymerized using hand held curing light after application of etchant and bonding agent.
Or
It is indirectly fabricated by dental technician in a dental lab after recording an impression and later bonded to the tooth/teeth using resin cement.
Porcelain Veneer: It may only be indirectly fabricated and later bonded to tooth/teeth surface.