Instruction manual says it resist against fire frost or shock

but then I went to a store and I ould buy "Frost Shield" "Fire Shield or "Shock Shield" I'm playing a breton and have the dragons ward which is just "Shield" I also have a spell with same effect as the greater power except i can cast as many time as I have Magicka points

and then theres the Santuary spell and what diffrences do the shields have vs the Resist Shock/Frost/Fire spells ?

Alright. Plain Shield acts the same as armour of the same level. Fire/Frost/Lightning Shield add resistance to the element in question, along with a small damage aura of said element (I'm not sure whether or not they add armour as well, but I don't think so). Sanctuary adds an extra chance for the enemy to miss you with physical attacks.