Age of Empires I and II Edit
In Age of Empires and Age of Empires II, attacks are applied against armor of the corresponding type. If a unit (or building) doesn't have a certain kind of armor, the corresponding attack will do nothing.
All units have two basic types of armor: melee armor (simply called 'armor') and pierce armor, which protects against most ranged attacks such as arrows. However some units, like the Throwing Axeman and the Mameluke deal "melee" damage (i.e. reduced by melee armor) at range. Depending on the unit type, the units may have at least one additional armor class, that is usually 0, so the bonus damage will fully apply (exceptions include the Cataphract and Condottiero).
Age of Mythology Edit
In Age of Mythology, there are three types of armor: hack armor (the same as 'armor' in the previous games), pierce armor, and crush armor. All units and buildings have ratings for all three armor types, given in percentages. The percentage quoted is the proportion of damage that is absorbed by the armor type.
For example, a Huskarl has 50% pierce armor, which halves the attack of an Arcus from 6 to 3. Units typically have 99% crush armor and much lower hack and pierce armor ratings, while buildings tend to have very high pierce armor and much lower crush and hack armor.