Armor, Shields, and Barriers are all "Defenses" that protect enemy (or Shepard's) healthbars from harm.
For Shepard personally, its largely irrelevant. Only two of the six possible Shepards have barriers (Vanguard / Adept), and none of the six have armor. For Shepard, Barriers and Shields differ only in color (purple vs. blue), as any armor piece that improves shields will likewise improve barriers.
Shepard cannot get armor at all; the Fortification power, despite claiming to be armor, is really a damage reduction, like Tech Armor (which, despite the name, is actually advanced shield tech).
For enemies, it matters a little more.
Armor
Indicated by a yellow health bar. Vulnerable to fire attacks, like Incinerate, and damaging biotic attacks like Warp and Reave.
Common to Cerberus Robots, Environmental obstacles, and reaper forces.
Slow firing weapons like Sniper Rifles, Heavy Pistols, and Shotguns tend to do better damage to armor.
Finally, Armor can be "weakened" by freeze effects and "penetrated" by weapon mods, or Armor Piercing Ammo. Incendiary Ammo, Warp Ammo or Armor Piercing Ammo, will all do additional damage to armor.
Barriers
Indicated by a purple health bar. Vulnerable to damaging biotic attacks like Warp and Reave, and by Concussive Shot.
Common to biotic users.
Fast firing weapons like SMGs and Assault Rifles do better damage to barriers.
Barriers also take additional damage from Warp or Disruptor ammo.
Shields
Indicated by a cyan health bar. Vulnerable to tech attacks like Overload and Sabotage.
Common to Geth, Cerberus, and certain Reaper forces.
Fast firing weapons like SMGs and Assault Rifles do better damage to Shields.
Shields take additional damage from Disruptor ammo.
Other than the fact that drones can move and turrets don't as Raven pointed out, here are the other major differences:
- The base drone damage is higher 40 to 25 and the drone gets more potential damage/shield upgrades.
- The turret can potentially get an ammo effect (AP or cryo) and a flamethrower (high close up dps)
- The drone can get an explosive effect when it dies and chain lightning (stun multiple targets)
The combat drone is offensively oriented and seems to be typically way better at distracting large targets than the turret as it can, and will, move in close and start zapping away. The turret is a lot more defensively oriented, designed to hold one spot on the battlefield or protect your flanks, etc.
Best Answer
The answer to your question is... yes and no.
As far as shooting through your teammates, you can do this -- somewhat. Depending on where you hit your allies, the shots will, in fact, go through. If you hit your allies in their arms or parts of their legs, the shots will go through and hit whatever is on the other side. Attempting to fire shots through their upper or lower torso will stop the bullets however.
To test this, aim at the ground through an ally's arm or the edge of their leg. You'll notice when you squeeze off a round, the shot will visibly ricochet off the ground. Depending on the surface, you'll see sparks or dirt appear. If you attempt to do this through their upper or lower torso, the "ricochet" effect will not appear, e.g. the round did not make it through the ally's model.
Assuming you can get an ally's arm or leg to line up between your gun and an enemy, your shot will, in fact, damage the enemy. It's just more effective to actually see what you're hitting, obviously.