In Skyrim there is a difference between magic resistance and elemental protection.
Magic resistance reduces the effect of magic by a certain percentage. This is not limited to damage. For example it also reduces the duration of Paralyze spells.
Elemental protection reduces the damage from a specific element (magical or non-magical source).
Different sources of magic resistance add up additively (25% mr + 40% mr = 65% mr). The same goes for different sources of elemental protection.
Magic resistance and elemental protection add up multiplicatively (25% mr + 40% ep = 55% dmg reduction).
Also magic resistance and elemental protection are capped at 85%.
So there is no 100% magic immunity in Skyrim.
The maximum damage reduction theoretically achievable is 97.75%, which you get when you have 85% magic resistance and 85% elemental protection.