How Do Fire, Ice, Poison... Shields Work?

So how do the Fire, Fire, Ice, Piercing, and Poison Shields work? 

Is it like the shield spell so a 2000 fire damage shield protects you for 2000 hit points and then it’s gone?

Or is it something like that reduces the total amount of damage each hit for the whole battle?

Its infinite the shield so it reduce the damage and it protect you for the whole battle

I agree with oPelle.

BTW I did some tests here, feel free to watch how much a 1000pts shield is protecting you :

Btw you might also take a look at this info: