The Necrons are one of the least played races in 40k. Most of this is due to the idea that their special rules are broken. While they may be more effective with 4th edition rules, they still work fine.
The Necrons are lead by the C’tan, the star gods, they were first discovered by the necrontyr. The Necrontyr had grown jealous of the old ones and staged a futile war against them. They turned their attention to the massive power held by the star gods. The Necrontyr offered the C’tan living metal bodies so they could interact with the material world. With these great powers, the Necrontyr successfully destroyed most of the old ones. The C’tan offered and tricked the Necrontyr into metal bodies so they could survive longer than their devastatingly short previous lifespan. However, the bodies took away their feelings, leaving them slaves of the C’tan.
Over the Years, most of the C’tan killed and consumed each other. Now it is rumored that only 4 C’tan remain. The most known to the other races are the Night-bringer and the Deceiver. On the board, both are a fearsome sight to behold. The Night-bringer can lay a devastating amount of high strength attacks upon its enemy. The Deceiver lays lighter blows, but can strike fear into its enemies, and create illusions that fool entire armies.
Necron Warriors are the heart of a Necron Army. They are almost identical to space marines, but carry dreaded Gauss weapons that can damage even the most armored titan. At 18 points apiece, they are expensive, but necessary for your troops. If 75% of your Necron units are killed, the remaining mysteriously vanish from the battle.
Heavy Destroyers and Destroyers are the big guns of the army. Heavy Destroyers find it easy to chew through all types of tank armor, while Destroyers can lay down a frightening barrage of firepower.
Wraiths and Scarabs make up the mobility of the Necron armies. Wraiths are devastating in close combat, and find no difficulty reaching enemies moving 12 inches a turn. Scarabs, are much like the Ork Grots. They are cheap, and next to useless. Often ideal for buying one some time.
Immortals, flayed ones, and pariahs are the elite forces of the Necrons. Immortals, the fist Necrons to bow to the C’tan, are equipped with devastating blasters. Flayed ones, the few Necrons that still have some consciousness, are terrifying in close combat. Pariahs, are the ultimate mix between immortals and flayed ones. With their Warscythe with a built in Gauss blaster, they can kill all who stand before them, at a Price.
Tomb Spyders when not in battle, tend to the Necron Tombs. In battle, they repair destroyed Necrons so they may fight again. Tomb Spyders close to other Necrons will be at a great advantage.
The Monolith. The Monolith is the single biggest, strongest vehicle in the game. Armor 14 on all sides, as well as the Living metal special rule, it takes a great deal to scratch the mighty tower. Every turn, the Monolith can teleport a Necron unit through its portal, have D6 heavy shots on all units near it, or throw out a devastating strength 9 template.
The Necron Lords are the HQ choice of the Necrons. Equipped with many choices of war gear, whenever fighting one you will not know what to expect. Anything from having a destroyer body, to resurrecting itself going back to full health.
One basic strategy is to create firing lines with your warriors. Line them up, then make a slow devastating advance towards the enemy. While this may work against some enemies, the Tau and Imperial Guard will shred you to bits before you can make it to the front lines.
Another strategy is to reserve your forces, then use a monolith to receive them. If this is used with a deepstriking monolith, you leave the enemy with little choice but to assault or fall back. Not only do you have the warriors shooting, but you get the added bonus of the monoliths deadly strength 9 template weapon or the Gauss flux arcs. The only risk with this is your warriors getting into assault with an army stronger than them such as the Orks or Tyranids.