Armor Save

Armour Saves are a mechanic from both Warhammer Fantasy and 40K, and represents the model in question's armor, as well as the durability of it. In both games, armor comes in many shades and sizes - Most standard troop armor ranges from a patch of metal on the shoulder to small vehicles, but though the function and look of the armor in question can vary, they function the same in game and are performed the same. Each model has exactly one armor save, and if it wishes to defend itself against a wound, armor saves are incredibly handy - Mostly because most dudes have one. Cover Saves are only gained in cover or when behind someone else, and in those cases, they are still pretty weak stuff, and Invulnerability Saves are even rarer, if better, as they can't be passed by powerful weaponry.

When throwing an Armor Save, you roll a single D6 per model - If the roll exceeds the Armor Save of the model in question, the wound is discarded with no further effect, but if the roll is below, the armor failed to deflect the bullet and the model takes the wound. Therefor, the lower the save, the more reliable it is.

That said, there are vastly different systems for how armor works in the two systems.

Warhammer Fantasy

I don't know squat about how this works. Please write if you know anything about it.

Warhammer 40K

The Armor Save behaves a bit different in 40k - Where your troops generally are lucky to have any sort of armor in Warhammer Fantasy, 40K has armor saves flying out the arse - Pretty much everyone has some kind of armor. Horde armies like Orks and Tyranids tend to have very low armor saves though, while more elite armies like Space Marines tend to have much better armor.

If you are new and just got your first Space Marine Codex, you might marvel over how your troops only die on 1's and 2's, believing that your new superhuman warrior models can wade out in a fire fight with impunity... And you would be right. If it wasn't for every single fucking thing in the game sporting multiple weapons who laughs at armor saves without any sort of trade off to the weapon or the bearer. See, where higher strength weaponry decreases the effectivness of armor saves in Fantasy, they are simply ignored through a special stat called AP that all weapons have. The number specifies how strong an armor the weapon will ignore, which means if a weapon like the Bolter has an AP of 5, it can completely ignore every armor save of both 6+ and 5+, completely negating the need for these fuckin' saves. Seriously, if a weapon doesn't have any kind AP it is deemed weak unless it spits out a large amount of decently strengthed (or rending) dakka. Do note that this is one often overlooked weakness of armor : It will statistically fail. No matter how lowly a weapon is, no matter how ridiculous or non-sensical it looks that it may get through your precious armor save, it will. This is why while you may think that huge blob of lasguns won't do jack to your terminators, it may just as well, with a bit of luck, a good order and the help of the rapid fire rule, wipe it.

So, armor in 40K is basically just a suggestion. No matter what, the enemy will have plenty of weapon to deal with heavy armor by simply ignoring it like Plasma Rifles, so don't be fooled by the Centurion and how it has an 2+ T5 save - If the enemy brings S 7 AP 2, there really isn't any reason to take them over another Tactical Squad. Also do note that the AP mechanic means gaining one point of armor is not simply 1/6 more chance of stopping your enemy's wounds : Not only will it do that, but against weapons that negated exactly your old armor save, you'll get a full new one. Also notice that, quite logically, best AP weapon get rarer and rarer. This is why armor gets exponentially better.

That said, here are the saves:

• 6+: A few plates here and there to stop the odd shot from killing the wearer. Used mostly by the Orks and the Termagaunts and Hormagaunts of the Tyranids, and could just as well not be there - Most weapons ignore them anyways. Other units like Kroot have this paltry save not to protect them, but as a final fail safe if they somehow end up outside of a forest. Everybody will laugh at it, especially weapons that will by definition be effective against their wearer - typically, low value / high number units - like flamers and grenades. Plus you can get better by standing in almost any cover. Don't count on it.
• 5+: What would be flak armor today - Can stop a normal bullet pretty reliably, but this is 40K were talkin'. Mainly possessed by the Imperial Guard and their fuckloads of infantry. Better than 6+. but doesn't do much to save your sorry ass if something wants you dead. Treat like 6+ - Better save but still ignored by and unreal amount of stuff (Pulse Rifles, Bolters, Gauss Flayers, etc). Still gives you a save against basic flamers, frag grenades/missiles and multi-lasers, so marginally better than 6+ in fact.
• 4+: Futuristic armour covering most of the wearers body. Usually not mechanical or technologically advanced, however. The 4+ is a save you can trust more reliably - Though still a toss-up, it is harder to ignore and a lot of sorta cheap troops have it, namely Fire Warriors, Necron Warriors and the Space Marine Scouts among others. Protects from bolters and their non-heavy cousins, which is a great plus. There are still things that ignore them, so watch out - You should be safe against most other troops, however.
• 3+: Power Armor that covers the entirety of the body with technologically advanced plate of awesome. This would be the sweetest save there were, as most Marines have it and others Elites tend to have it too, like the Crisis Suits and Immortals, if there weren't so fucking many "Marine Killer" weapons around. It certainly has the best usefulness ratio as it will protect of most if not all non-vehicle-mounted or non-specialised weaponry, while the next armor ( 2+ ) will probably not fare any better against those weapons that do ignore 3+. Will defend your dudes reliably, but keep away from Plasma and anything that ends with -cannon.
• 2+: The grand daddy of Armor Saves, this is heavy Power Armor like Terminator Armor and the armor of heavy walkers. Again, fuckin' a if there are no AP 2/1's around, but you'd be lucky to encounter that. Combined with the usually low number of troops carrying these kinds of saves, make sure you know how to keep the big guns from them - Can you manage that, then you are golden.