In the core rules shields provide a fixed amount of protection, and helmets are figured into the protection armor provides. This is a bit simplistic, and is replaced with the rules below.
Their primary funktion is to provide extra amour to a vunerable area, the head. It provides its damage-reduction die to all hits.
Shields provide a defensive die (eg. d6) to armour, when weapon-skill roll was sucessfull. This does not include critical strikes.