Armor: Armor is used by wearing it. Most armor will automatically appear in the inventory of the bodypart or bodyparts that the armor covers, but _some_ will allow you to choose which part to wear it on. Typing in wear greave on right leg would be acceptable but wear helmet on right arm would not.
Some armor will cover more than one part of your body. For instance, if you wear a chain hauberk, it will protect your arms, legs, and body and would therefore appear in the inventories of all those parts. Some armors can overlap (you can wear a hauberk, as well as greaves, giving higher protection to your legs, for example); some might not (certain hauberks cannot be worn with a breastplate, for example).
Armor can slow you down, making it easier for you to be hit, but it absorbs damage, having a damage multiplier between 0 and 1.
Armor can be damaged, and even destroyed if it absorbs too much damage.