Your AC is your overall avoidance, unlike the skill evasion, you do not have to spend most of your round to use it. It is a passive bonus, it is far from as high as evasion, and doesn't cover your ability to leap out of the way from certain things however. While it might be totally awesome and at least 20 % safer to strut around in a power armor like the Steel Rangers, you might notice the long list of armor check penalties such an outfit would apply. While that resistance to explosions will save your flank in the case of one, you will find yourself quite unable to make a nimble side-step, run away from that large whatever the fuck it is with huge claws and teeth, or hit the deck if a low-flying very sharp or hard object were to pass by. So just keep in mind that, yes, it is worth to have armor (without a doubt), but find your balance in protection and how much of your other abilities you are willing to sacrifice for it.
AC: Armor , and your agility makes up your ac. (unless you take perks like: Kamikaze it is most likely this number). What exactly does AC do you say? Unlike other systems where it's something you either try to roll above, or something that reduces damage you take, the AC in this system does both, and neither. AC in this game reduces your opponents roll to hit you. Lets say Bob the raider tries too punch you in the face, now you don't like being shot in the face. But luckily, you are wearing a matrix armor (patchwork of several scrap pieces) that gives you 15 ac, and your Agi is 10, so that makes a total of 25 ac. Now, Bob the raider attacks you with a power-hoof, he gets a rolls 44, and his unarmed is 62, now normally he would have hit you in the face as his roll is successful. But your AC reduced his roll by 25, (increasing the result) leaving his final result on 69 instead, thus making him fail the roll and miss.
If using evasion however, you just roll an opposed check with no benefit from AC, and the one with the highest success wins the check. You must however be ready for the attack/explosion/etc, and have spare action points. Spare action points can also increase your AC by 3 per unused point. Some perks are also able to affect the number of both AC and evasion. Never both of them simultaneously, but you get the idea. (Althou, unless you count perks that gives your more AP, but that's beside the point.) Friendship points can affect the result of either of these results. But then again, they can pretty much affect every single type of roll, effect, save, event and whatever else that exists.