Power armor is exactly that. It's a powered suit of armor. Medeival knights were the tanks of their day, but you could take them down with a good lance to their neck. Un-horse them, then hit them heavily enough with hammers to dent the armor, and, well, they're out of the battle, if not dead.
Space Marine power armor is composed of ceramite and plasteel. Games Workshop's own made up variants of ceramic alloy and plastic alloy. An alloy is any combination of a metal and a non-metal. The theory behind ceramite and plasteel being, I think, that ceramic armor plates are known to be some of the toughest s*** around today, and some secret steel alloys are even tougher. Add to that, plastics are so ubiquitous, the STCs produced in the 20-30k era probably included a dozen different ways to accquire plastic's ingredients, make plastic, recycle plastic and re-enforce steel with plastic.
Erego, you have ceramite and plasteel. To give this some perspective. Look up dragonscale armor. It's a relatively new armor invention (don't confuse with dragon skin, this is spider-web protein spun into a silk-like fabric, which, in thicker layers, makes for excellent bullet-stoppage). Dragon scale is half-inch thick ceramic disks, overlaid in a scale-mail fashion of armored vest, with kevlar backing. (edit: Supposedly,) the dragonscale has been tested against abrams (edit: tank) cannons, and the cannonshell (edit: is alleged to have) stopped without penetrating the kevlar. I think it would still pulverize your innards, but, I don't know what happened to their ballistics-gel dummy, when they did this test, aside from I think it flew backwards several dozen yards.
Now, in 40k, space marines wear an armor suit of plate mail composed of inch-thick layers of ceramite, so imagine dragon scale, but harder, or perhaps as an alloy, it has some bend to it, so it doesn't shatter. I don't know. Underneath this outer layer of inch thick ceramite is usually a thin layer of plasteel, to re-enforce (though in some variants, I've read the plasteel is between half-inch layers of ceramite) the ceramite.
Beneath the outer layer of armor plating, a space marine has an entire suit of fiber bundles and flak-resistant fabric, along with several black-carapace interface points, which allow the space marine to feel what the armor feels (heat, cold, bullets and lasgun impacts), he can assess where the damage is hitting hardest, and turn his body so less damaged armor takes the brunt of the force. Space marine shoulder pads are also huge. They spread like a shield on either side of the marine, and I remember reading they are twice as thick as the rest of his armor, so... two or two and a half inches of armor plating over fiber bundles over kevlar over black carapace. The black carapace, while being a nerve-system relay for the armor, also functions as an inner armor suit, as even without the power armor, the black carapace protects him from simple knife and small arms fire, to some extent.
Space marine power armor can shrug off all but the heaviest weaponsfire, but does not make them invincible. It just makes them damn hard to kill. Don't get me started on terminator armor.
Edit: Replied before realizing how long ago that post was. Hope it was a helpful explanation though.
Edit2: Souls of slain primarchs are probably just like those of the ascendant primarchs. Living the life of a daemon prince in their respective pantheon (except that, well, each chaos god looks poorly on the concept of worshipping all four, and tends to try to rip a portion of your soul for themselves, but I'm sure for primarchs, the four would make exception and grant them... boons).
Last edited by Belthazor Aurellius; 08-04-10 at 12:47 PM.