They're basically the "space undead".
Most Necrons are Humanoid robots with a largely skeletal appearance. They are made of a special material known as necrodermis, which is a type of living metal capable of repairing itself. Each is controlled by the consciousness of a former organic being bound inside that has been rendered devoid of emotion in it's mechanical form.
The Necron race includes various other designs beyond the basic humanoid form, including insectoid Tomb Spiders and Scarabs and Pariahs, which are cyborgs made from genetically modified humans with an inverted Warp Soul. The effect of which is the ability to strike crippling terror into the most hardly disciplined soldiers.
The Necrons were originally a humanoid organic species that existed about 60 million years ago known as the Necrontyr. The Necrontyr were native to an irradiated planet and as such suffered from short life spans despite being technologically advanced. They were a militaristic and expansionist species that came into conflict with another ancient race known as the Old Ones. During this conflict, the Necrontyr discovered the C'tan and constructed physical bodies of necrodermis so they could communicate with them. This allowed the C'tan to develop a taste for the energies of sapient beings over the tasteless energies of stars, while the Necrons turned to worshiping the C'tan. The new hunger brought the C'tan into conflict with the Old ones, and the C'tan offered the Necrontyr the means to win the war by transferring their minds into necrodermis bodies, converting them into the Necrons. After the defeat of the Old Ones, the galaxy was ravaged by the Enslavers, and the C'tan and the Necrons went dormant. While a few of them did go about covertly doing minor tasks, only recently have the Necrons begun to awake in significant numbers.
Besides their subservience to the C'tan, individual forces of Necrons are led by figures known as Necron Lords, which display considerably more autonomy then the rank and file of the Necrons.
- Necron Warriors: Warriors are the common footsoldiers of the Necrons.
- Necron Immortals: Immortals are shock troops that are larger, stronger, more durable, and more heavily armed than their Warrior counterparts.
- Necron Destroyers: Destroyers are support units equipped with anti-gravity flight systems and heavy weapons.
- Necron Lords: Lords are Necron officers that lead military units.
- Necron Monoliths: Monoliths are heavy vehicles that incorporate both weapons and teleportation systems to bring in reinforcements from remote locations.
Only comparatively recently have the Necrons begun to reactivate, and large numbers of them still remain dormant on various tomb worlds. Never the less, the Necrons are definitely a force to recon with; their technology is likely the most advanced in the Warhammer 40,000 universe, and they have the best FTL of any WH40K species. Necron Gauss weaponry is extremely powerful, ripping apart a target at an atomic level and teleporting said destroyed matter into the weapon, a standard Necron Gauss flayer can fairly easily destroy a Space Marine. Even when a force is victorious over the Necrons, they simply teleport themselves and any disabled units back to a secure location, where they are capable of salvaging even severely damaged Necrons due to the self-repairing nature of their Necrodermis construction.
The Necron warships are the fastest in WH40k, in both FTL and STL modes. Their interstellar drives can cross the Milky Way galaxy almost instantly. Necron spacecraft have only been defeated by superior numbers.
Above all else, the C'tan despise the Immaterium and the things that come forth from it (such as Chaos). The long-term goal of the C'tan is to isolate the physical world from the Warp via a megaproject known as The Great Warding.
A big fan of the Necrons and their masters on SD.net is Necronlord.