Still very new to SC - hoping somebody can clear something up for me...

What is the difference between the shield upgrade and the armor upgrade for protoss? I know that shields will apply to buildings as well - but ignoring buildings for a second - what is the difference in the way that they affect units?

My understanding is that if you upgrade the shield from 0 to 1, then get attacked by somebody who does 10 damage, you will take 9 instead. If you have upgraded your armor to 1 as well, does that mean that you take 8 damage? Or does the armor only start going into effect once your shield is down. And if so, doesn't that make it not very useful?

Thanks in advance

Damage dealt to your shields will be mitigated when you upgrade shields, damage dealt to your health will be mitigated when you upgrade a specific armor-type for the unit in question. In most cases, your health cannot be damaged until your shields have been depleted. The big difference between these is that shields regenerate and also effect a vast array of your forces (buildings, ground, and air units) and not just a specialized group of them. However, the amount of shields that a unit has is typically less than the amount of health, so the upgrade that will benefit you more can vary depending on your play-style.

I like upgrading shields when I know I will have a varied army, as it will help my army as a whole and not just the individual classes of units. It also helps me when I am switching techs so that I am not as behind on upgrades.