StarCraft II - The New Armor System
If you played any Blizzard RTS since StarCraft, you've experienced
Armor Types. Using a unit whose weapon is strong against that
type of armor renders far more damage than if you shot at the target
with a weapon of a different class. Understanding these, and
some of the traits associated with them is imperative to winning at
StarCraft II. No longer are there penalties for attacking,
only bonuses thankfully!
Basics of Light
Light units tend to hit quickly and be quick themselves, however they often are small and fragile. Using weapons strong against light will bring them down in the blink of an eye, rather than having a chance to react with most Armored units. Units that you wouldn't expect to be light are all worker units, and all mage/ability caster units regardless of their size except for the Infestor.
Most rushes will consist of basic starter units, which are all Light. Since these units will be seen as either meat shields (Zealots) or fast damage (Zerglings, Marines) you'll need to be ready for them at any given time. Fortunately, their counters are devastating.
Terran - Hellions devastate group of light units, and out-range them all. No contest if you can control them and keep them alive. These can do massive damage to light, and low to everything else.
Zerg - Roaches are good due to their armor and high damage, slow attack speed of play. You want to hit and run and keep the enemy from being able to surround you with their superior numbers of melee units. Banelings are another option, with a very strong bonus against light units and an area of effect attack that demolishes groups of the little guys. Corruptors fare well against light air due to their low damage against the Corruptor's tough defenses.
Protoss - Archons are a solid counter to grouped light units since they're all biological for the most part. However, Colossi are a much simpler unit to use and have more use throughout the game... if not countered. Sentries can be used to split up the hordes to make them easier to deal with.
Basics of Armored
Armored units are big. They have some of the slower shooting speeds, and higher damages of the game. However, Terran and Protoss both have powerful anti-armored counters. These units form the bread and butter of your assault forces, but overdoing it can lead to an easy counter if it's scouted.
Armored units are all over in the end game for all sides. Remember that if it's not light, it's armored. Most air units and almost all higher tier units are considered armored.
Terran - Marauders are where it's at for cheap, cost effective anti-armor. Tanks in normal mode have a big bonus as well, and actually do some of the highest damage in the game to armored units over time. Vikings rule the skies with their anti-armor missiles.
Zerg - Spine Crawlers get a boost, but those are impractical as counters. Use hordes of Zerglings to overwhelm them--armored units often shoot slowly. Back them up with an Ultralisk or two, they now do massive damage to Armored units with less upgrades needed to be effective than before.
Protoss - Stalkers get a slight bonus, but if you're dealing with armor, Protoss has the best in the game at it. The Immortal does 150% more damage to armored units than it does to light units, and often times their attacks will bounce off the Hardened Shields for far less damage. Void Rays, once fully charged, melt anything Armored in fractions of a second. This includes land or air!
Does it matter or can I just mass Zerglings?
Armor-type counter in this game range from a slight bonus to "don't bother" territory. Units like Stalkers and Vikings get a small boost to their attacks, but units like the Immortal and Void Ray get an absurd bonus when attacking their favorite targets. If you're caught massing a single unit type and your opponent has time to assemble the counter, it will be painful. This is nice, because it encourages combined arms even more than the original did! So keep these armor bonuses and weaknesses in mind as you head out into the battlefield next month, and from launch on!