Armor

Armor is a unit property which reduces incoming damage by a set amount per hit. Armor may be upgraded at certain structures.

Armor can make a significant difference in what units counter what. Units that do low damage are most penalized by attacking a target's armor. Marines and zerglings are thus most affected by high armor. Ultralisks are a good counter to MnM for this reason.

Armor upgrades are also very significant. Upgraded weapons can reduce the number of hits it takes to kill a unit, and upgrading the unit's armor in response can counter this. For example, zerglings die to zealots in 2 hits instead of 3 if the zealots have 1 weapon upgrade and the zerglings have no carapace upgrade.

All attacks will do a minimum of 1 damage, even if the armor is higher then the attack's damage. However, some special attacks ignore armor, and all spells ignore armor.

StarCraft II

In StarCraft II, spell attacks ignore armor.

Armored is a unit type. Some units gain bonuses towards light or armored units.

Wings of Liberty

In StarCraft II: Wings of Liberty, each armor upgrade increases hit points by 5% when the vanadium plating upgrade is researched in the laboratory.