Well, since most have focused on the back story of the imperium and space marines, I'll tell you a basic summary of the Necrons.
They were a race with a super radioactive sun, and that gave them painful, short lives, when the old ones (A really nice, very long living race) wanted to help them, the Necrons took it as patronizing, so they got angry at the old ones. this led them to study their sun, and they found an ethereal being eating energy from their sun. They brought it to them and gave it a body of living metal (these sun eaters are called C'tans) It began to eat the Necrons, because they were more tasty than the sun (Not a joke, thats why) the Necrons promised to worship it as a god if it would stop, and it did, it also made them take metal bodies like the ones they had gave it, however, there were more C'tans who wanted in on the action, and so, that C'tan gave worshippers to the other C'tan, eventually though, only four C'tan remained, because they had all eaten eachother for more power. The remaining four are called The Void Dragon, The NightBringer, The Deceiver, And The Outsider. Now, the Necrons worship them by attempting to destroy all life in the universe.
And while I'm at it, Tyranids, since they're pretty quick,
They're a species with a hive mind that devours all things biological (Plant, animal, everything) and complely destroys all life on planets. So far their have been three majour attacks on the Warhammer galaxy by the Tyranid Hive fleets, called Behemoth, Leviathan, and Kraken. The Imperium has so far introduced only one contingency plan against them. If a planet ever becomes hoplessly lost to the Tyranids, destroy it.