Armor
Armor is very important since it protects its wearer from physical blows and limits the amount of damage one takes. The lighter a set of armor is, the less it deflects but the more flexibility it has. The heavier a set of armor is, the more it deflects but it lacks in flexibility.
Armor can be made "Superior Quality" by spending an additional 300 G on it. Superior Armor decreases the Armor Penalty by 1. Magical armor is already considered to be of "Superior Quality" and thus the bonuses don't stack.
Armor grants a bonus to Reflex Defense, however some armor affects Fortitude Defense (example: a suit coming with life support). This is most common among futuristic armor, or magically enhanced armor.
Spell Failure applies to characters who wear armor they aren't proficient with, and try to cast a spell. Mages, for example, aren't proficient with light armor. However if they become proficient in it through the use of a feat, they ignore the spell failure %.
Armor Pieces
In addition to armor protecting you with its standard Armor Bonus which applies to Reflex Defense, armor can also protect various parts of your body as well. The protection granted varies depending on the type of armor piece worn. These pieces will be separated into four parts: Helmets, Arm Pieces, Hand Pieces, and Leg Pieces.
Individual armor pieces which are made of special materials don't get any mechanical benefit from being made of that special material. If you buy an armor which includes these armor pieces and get the armor made of a special material (example: buying a steel fullplate) the individual pieces are counted as being made of that material as well at no additional cost. This can be done later as well, buying a fullplate and choosing to get it upgraded mid-session into a mithral fullplate. All of the individual pieces can be upgraded along with the fullplate at no cost, even if the individual pieces didn't come with the fullplate. This is because a mithral pauldron would be just that: a mithral pauldron. It'd still possess the same cost, armor penalty, weight, and bonuses. The exception to this rule is Gauntlets which can be enhanced with Weapon Enhancements and can receive a Magic Weapon Bonus, for this purpose Gauntlets count as weapons, not armor.
Certain armors come with these armor pieces already (without increasing the cost, armor penalty, or weight while they are worn). The armors will be listed below and what they include. Removing the individual pieces that already come with the armor does not reduce the armor penalty or weight.
Chainmail: Gauntlets.
Breastplate: Gauntlets and Greaves.
Powered Battle Armor: Standard Helm, Pauldrons, and Greaves.
Half-Plate: Standard Helm, Gauntlets, and Greaves.
Fullplate: Standard Helm, Pauldrons, Gauntlets, and Greaves.
O-yori: Standard Helm, Vambrace, Gauntlets, and Greaves.
Heavy Powered Armor: Great Helm, Pauldrons, Gauntlets, and Cuisses.