All armor has an armor value referred to as its AV. When worn, this gives you an armor class (or AC) of 10 + AV. These numbers may seem low for people familiar with other systems, but that's because blocking is a core part of the game. Some abilities such as Unbreachability will improve a creature's AC regardless of what armor they are wearing. In Teriock Online, AV does not stack. This means if you have an ability like Natural Hide and then don leather armor that your AC does not change.
Many classes also have a maximum AV they can wear.