Health is good. Lose it and you die. Dying is bad. But having high Health won't get you through every challenge on its own. To make your Health points even more precious you're going to need to mitigate incoming damage, and to do that you'll need to rely on a variety of defenses. There are several statistics which reduce damage; physical damage is mitigated by your Armor Rating and Elemental Resistances and Magic Resistance combine to help you weather elemental damage and magic damage (there is much overlap between the two). Furthermore you have Magic Absorption and various other perks that help deflect damage, but this section will focus on helping you understand Armor Rating, Magic Resistance, Elemental Resistances and Magic Absorption.
In Oblivion and Fallout 3, determining your damage resistance was pretty straight-forward; you had an armor score, and that was how much damge you resisted from every attack, capping out at 80%. In Skyrim, the damage resistance cap is still 80%, but now you have this brute number hidden behind a fancy "Armor Rating" stastistic.
The more/better armor you have on, the higher your Armor Rating. Due to how damage resistance is calculated, however, the more pieces of armor (including a shield) you're wearing at once, the lower the Armor Rating score you need to hit that magical 80%. If you're wearing four pieces of armor (helmet, armor, gloves, boots) and a shield, you only need an Armor Rating of 542, but you won't always have a shield equipped - dual wielding, casting spells and archery are all aspects of the game that interfere with this. The more common number to shoot for then (and the goal put forward in this guide) is 567 - the Armor Rating you need to reach to hit 80% damage resistance with four pieces of armor and no shield.
Given how much you can improve armor via the crafting skills (Alchemy, Smithing and Enchanting) it's fairly easy to hit this mark. Four pieces of Dragonscale Armor can get you up to 1000+ Armor Rating with only one perk invested into "Agile Defender", for an example of how easy it can be to reach the cap if you follow this guide's build advice. Even lesser suits of armor can, when properly improved (and perhaps with a few extra perks thrown in) reach the cap. In any event, the number to shoot for is 567, so keep that in mind.
Note: You can tell what count as armor and what doesn't by the words "Heavy Armor" or "Light Armor" in the upper left of an item's description. If it's not there, it's not armor. With no armor on at all you'll need an Armor Rating of 667 to reach 80% damage resistance.
Elemental Resistances come in three flavors; Fire, Frost and Shock. Nords start out with a 50% resistance to Frost, while Dark Elves (Dunmer) start out with a 50% resistance to Fire. Unfortunately, given the limited number of items that can bear these enchantments (boots, ring, necklace, shield), most characters will have to settle for 56% (the maximum amount you can get from enchanting your own gear) in each, which is well shy of the 85% maximum. Fortunately, however, there are other ways to shore up your resistances.
Magic Resistance is more straight-forward than Armor Rating, coming in straight percentages. There's only one type of magic, too, and since most sources of elemental damage are magical (including dragon breath!), Magic Resistance is your best defense against elements, too. The effects are fortunatel cumulative, meaning that if you hit the cap of 85% Magic Resistance you'll have that much damage reduced, then the remainder will be further reduced by your specific elemental resistance, if applicable. So if you take 500 damage from a source, and have 85% Magic Resistance that'll be reduced outright to 75 damage. If you then have 56% elemental resistance that'll be further cut to 33 damage. That 500 damage doesn't look so scary anymore, does it?
Unfortunately, Magic Resistance is even harder to enchant on your gear than Elemental Resistance, applicable only on shields (which you won't always be using), rings and necklaces, and in inferior magnitude than Elemental Resistances.
Fortunately, there are other sources, the best of which is Breton Blood, which grants Bretons a permanent 25% Magic Resistance. On top of this you can permanently get 15% Magic Resistance from the "Agent of Mara" active effect, obtained by completing the quest "The Book of Love" . The Lord Stone will further grant you 25% Magic Resistance while active, bringing your passive total up to 65% (if you're a Breton) or 40% (if not). In the Alteration tree you'll find the aptly named "Magic Resistance" perk, whose three ranks can bestow you with a total of 30% Magic Resistance, although it'll cost five perks to do this, in total.
Altogether this allows Bretons to get the "Magic Resistance" perks in the Alteration tree and enjoy a passive, permanent 70% Magic Resistance, whichs is frankly good enough for most encounters (which paired with some Elemental Resistances, anyways). Everybody else should supplement their Magic Resistance scores with The Lord Stone.
Magic Absorption is a bit of an odd statistic, mostly because it's poorly-worded, in game. For example, the "Atronach" perk in the Alteration tree reads "Absorb 30% of the magicka of any spells that hit you" . Fair enough, so it a spell that costs, say, 30 Magicka hits you, you'll absorb 10 Magicka?
Instead this effect has the stated chance to negate all damage from said spell, and if successful, it'll add the full Magicka cost of that spell to your Magicka pool. Atronach's 30% Magic Absorption is nothing less than a 30% chance to negate any spell. Best of all, it's calculated before Magic Resistance and Elemental Resistance, providing a third aegis protecting you from nasty spells.
Unfortunately it's terribly uncommon; you won't be able to enchant it onto gear, and there are only two ways to get it more or less permanently; the "Atronach" perk, and The Atronach Stone. Still, these effects combine to give you 80% Magic Absorption, which is damn near proof against most magical effects in the game, and in combination with a high Magic Resistance score and Elemental Resistances, it'll make enemy spells almost a non-issue.