Necrodermis is the mysterious "living metal" (even though it comes from Greek roots meaning "dead skin") from which Necrons and their constructs are made. It is more resistant to damage than most materials known to the Imperium and is capable of repairing itself to a certain extent -- melta weapons and other "overkill" weapons will put them down for good, but many Imperial Guardsmen have filled Necron Warriors with lasgun holes and knocked them down, only to see them rise up whole.
As a whole, Necrodermis borders between Flak Armour and Carapace Armour. Something analogous to the Eldar's Mesh Armour. Whilst it can take more direct hits from small arms than Flak Armour, heavier weapons like Bolters would most definitely punch through this material (Although it depends on how thick the Necrodermis is). The main catch of course, is that it could self-repair over time as previously mentioned.
The shape of a Necrodermis can also be modified at will by its C'tan inhabitant, leading to vast differences between the bodies of each Star God. This also allows the spontaneous formation of cutting and slashing weapons as required.
The material that makes up Necrodermis is the aforementioned living metal (Nanomachines perhaps?). The Necrontyr originally invented living metal for use in constructing their ships. It is most often associated with the skin of the C'tan as well as the bodies of the Necrons and their amazing regenerative abilities.
The hull of all Necron vehicles, including the Necron Monolith and the Necron Pylon, is also made of this living metal, where its adaptive and regenerative capabilities render powerful weapons such as Meltaguns, Lascannons and Power Fists almost obsolete. It also has the curious effect of interfering with any attempts at warpcraft. While certainly not to the same degree as blackstone, on the few occasions that daemons or warp-inclined Chaos Space Marines have come into contact with the Necrons, it's oft mentioned that the necrodermis seems to play havoc on any warpfuckery.
Ferrus Manus's iron hands might (or might not) be made out of Necrodermis which he got from murdering a huge worm-like creature made out of "some kind of metal" (sounds familiar) with his BARE Hands. Actually he single handedly drowned the creature in a pool of lava and the molten metal that dripped from the creature got fused with his hands.