In many R.P.G. systems the designers have decided on simplicity when deciding on how to define weapons and the type of damage they can cause, or they simply stated a damage amount.
I have decided on a more complex system to "I hope" capture the feel of the real weapon in combat. As an example think of a standard sword, like a rapier. You can stab with it, slash, chop (if your desperate) and use the pommel to bludgeon your opponent. Some of these attacks result in better damage compared to other attack types. This reflects the weapons design and how it was meant to be used. So the rapier does better damage with a piercing attack than a bludgeoning attack. Yet a Battle Axe in comparison is designed for chopping and will do more damage with a chopping attack and is poor in dealing piercing damage.
Armor is defined as a trade off between mobility and protection. Armor protects the wearer from damage, it does not increase their difficulty to be hit. The most protective armor is bulky, stiff and heavy. This combination reduces an armored characters agility in combat but grants a artificial damage reduction. The different types of armor are to provide a varying degree of protection vs agility.
Nature Armor is a part of the species initial design or earn from leveling in specific classes. Armor can be worn over the Natural Armor of a species. Both provide protection from damage but only the Armor, not Natural Armor, will affect agility in a negative fashion.
Shields have long been used in combat to protect the user from both ranged and melee attacks. A shield is a versatile defensive barrier that can do more than just protect a user. Most shields permit the use of the shield bash attack and the defensive deflection combat action. Small shields also allow the user to make a defense weapon grapple.
Helms provide simple protection from head trauma like, daze, stun, and so on. You may only use helms for your size category. Larger ones are to loose and smaller ones will not fit. Even a helm in your size category may not fit as it may have been made for a race without the horns you have, or the head is oddly shaped.