Carapace Armor
Carapace armour is basically Flak Armour LV2. A bit tougher, and a bit more expensive. Usually given to commissars and stormtroopers. 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.
It is expensive but still cheap enough that Krieg makes use of it in its own version of grenadiers who go in first and are expected to die first (along with hot-shot lasguns) so perhaps it isn't all that expensive after all and everyone is just plain too stupid (stingy) to realize that.
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(yeah it's as lazy as it sounds) and Camo Cloaks. To make themselves super stealthy instead of giving extra protection. (Unless you're Raven Guard who sneak around in full Powered Armor to get best of both. Though 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.
Deathwatch Scouts have further modifications. Upgraded with gel-passages, a re-breather system that contains both oxygen and water, auto injector gloves, a vox system, and a special coating. For enhanced mobility and extended operations while sneaking explosives or teleport homers into the homes of unlucky xenos.
Void Armor
The original version of Carapace Armor. A step down from Power Armor. Void Armor were standard issue protection for the Solar Auxilia. As the name implies this armor is the grim dark future version of a space suit. Meant for combat in conditions that even most modern type Special Forces unit would find difficult to operate in. Unlike Carapace Armor it seals off most of the wearers body as void armor comes with a nifty looking Steam punk style helmet.
Fluff says it was made by Saturnyne Domains. So now it's nearly impossible to create by the 42nd Millennium. As 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. Giving extra protection from Flamer style weapons and most long explosives. These were worn by Veletaris Storm Sections and Auxilia Flamer sections. So this implied 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 that much of a bad idea.
Though it might seem right out insane for the average human to shoot it out with a Space Marine. Volkite Chargers and Flamers will make it absolute hell for anyone without a defense comparable to Terminator Armor to stick their Chainsword into them. By the time a loyalist or traitor got themselves close enough, most of his buddies were turned to ash or right out vaporized. Finding it difficult to penetrate the armor of a single Veletarii after and than he would cooked alive by his squad mates. So even the most assault focus of Astartes 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.