There are various different armor sets that you can pickup throughout your travels. Each piece of armor will provide a certain level of defense based on some particular elements of defense type. For example, some armor may be well suited to defending against fire and others might be better against ranged shots. The game allows you to carry a few different armor sets on you at any time, this allows you to focus on particular defenses. If you are about to engage in a battle with some enemies that shoot fire, equip the armor that is tailored for this use.
Just like weapons, it is possible to add some runes to enhance the effects of your armor. Based on how rare the armor is, you will have a certain amount of free slots to apply the runes to. As I mentioned above, it’s a good idea to try and have some armor that is specialized, but no harm having an all round armor set too.