Carapace Armor
Carapace armour is basically Flak Armour LV2. A bit tougher, and a bit more expensive. Carapace Armor is usually given to commissars and stormtroopers, but is sadly a step down from Power Armor. It is currently the best armor that non augmented humans outside of the Sisters of Battle and Inquisitors can get their hands on. Rarely it will also be worn by Lost and the Damned or a well equipped PDF. Generally speaking, this is the most protection an ordinary man will get to use unless they get extremely lucky/prove useful enough to wear some form of powered armor.
Like Power Armor it has Ceramite or armaplas (perhaps plasteel, armaplas is never really explained) molded into armor plates. This also makes it much heavier and will fatigue the wearer much faster than flak armour or power armour given that it usually lacks any kind of assisted movement systems, though that isn't always the case as semi-powered examples do exist. Usually, soldiers issued carapace armor are elite units and given light genetic enhancement to bear the weight without problems.
The protection provided by a set of carapace armour varies a lot from story to story, as you'll frequently hear of normal bolter bolts punching clean through power armour (at close range) and thus should go through carapace like it wasn't even there, however on the tabletop now carapace offers some protection from the likes of heavy bolters, power mauls and autocannons, and to a lesser extent even Hellguns, Inferno Bolters and Sternguard Bolters.
The largest problem with wide-scale deployment is one of cost. While some regiments, such as Krieg, can afford to fabricate and maintain carapace armour for its elite grenadier units (and tanks, heavy tanks, and super-heavy tanks and other giant vehicles in massive quantities), mass deployment becomes another issue (because the Imperium has yet to put its worlds on a war economy footing, so it's always lacking in everything its military needs. Seriously, a Hive World dedicated to war churns out continent factories of tanks and armor.). To whit, carapace armour costs roughly the same to put on a guardsman as it does to shuttle in a fresh guardsman. While in elite units the RoI for combat experience can be worth the added cost to equip, the general opperendus of the Guard is to ship two men instead of one well-armoured one. Of course, if you armored the Guardsman he wouldn't need to be replaced in the first place, but the Administratum is not known for being remotely competent.
Variants
Scout Armor
Per usual any version that normal humans have, Space Marines will have a better version. Instead of Power Armor, Neophyte Astartes who survive the Geneseed process become Scouts are given Scout Armor and Camo Cloaks. The purpose of this armor is to make themselves super stealthy instead of giving extra protection (Unless you're Raven Guard or their Successors who sneak around in full Powered Armor to get best of both. However there is a fluff reason for that.) Even Sergeants and Veteran Sergeants (who are members of the First Company) wear it when they volunteer to train the newbs. As unlike certain chapters, you don't become top dog if you act like a total douchebag (in an ideal Imperium of Man, anyway).
Deathwatch Scouts have further modifications, with gel-passages, a re-breather system that contains both oxygen and water, auto injector gloves, a vox system, and a special coating. These upgrades deliver enhanced mobility and extended operations while sneaking explosives or teleport homers into the homes and HQs of unlucky xenos.
Void Armor
The original version of Carapace Armor. A step down from Power Armor, Void Armor was standard issue protection for the Solar Auxilia. As the name implies, this armor is the grim dark future version of a space suit. Void Armor is meant for combat in conditions that even all but the most specialized units would flounder in, like the vacuum of space or extreme exo-planetary conditions. Unlike Carapace Armor it seals off most of the wearers body as Void Armor comes with a nifty looking Steam punk style helmet. Void Armor had low level self repair systems that worked against small amounts of penetration and lacerations, while being resistant to radiation (due to space having Cosmic rays and all that other stuff) and thermal effects.
Fluff says it was made by Saturnyne Domains, so now it's nearly impossible to create by the 42nd Millennium. If the Imperium bothered to store the STC printouts, we might still be seeing this armor in wide use today. Interestingly (or perhaps not, thanks to Wardian fluff), the Grey Knights now call this region of space home.
Reinforced Void Armor
An enhanced version of Void Armor. The Reinforced variant makes an unaugmented human almost as hard to kill as a Space Marine, thanks to its extra protection from Flamer style weapons and most long range explosives. These were worn by Veletaris Storm Sections and Auxilia Flamer sections. This creates the implication that Reinforced Void Armor was meant for close quarter fire fights. With the number of Xenos who love running head first into the battle lines or ambushes in dark corridors, an enhancement of Void Armor isn't a bad idea.
No sane human being would get into a firefight with a Space Marine, but the Horus Heresy made for an insane time period. As such, the Auxilia had ample access Volkite Chargers and Flamers. This would make it absolute hell for anyone without a defense comparable to Terminator Armor. By the time a loyalist or traitor got themselves close enough, most of his buddies were turned to ash or simply vaporized. Even if an enemy could survive long enough to close the gap, that enemy would be easily vaporized by the nearby allies of whoever was getting attacked in the first place. Without factoring in the difficulty of even penetrating reinforced Void Armor to begin with, it was clear that taking on these guys in melee combat was a sheer impossibility unless you were an Astartes or in a massive horde. Even then, the most assault focused of Astartes would be reduced to slinging Bolter rounds, energy beams, plasma and grenades against squads of normal humans like it was World War One in space. These brutal firefights between Auxilla and Space Marines on both sides would have most likely been common during Horus's rebellion, where average shmucks in barebones half flak armor would be turned to soup in short order. Assuming any reasonable number of Auxillia turned traitor. Since they're from Terra, they probably didn't much like how most Terran-born Astartes remained loyal.