okay, 52 isn't really necessary, although reading issue 52, or just understanding what happened in Week 52, would be a good primer for this.
Here's the Big Scene DC Timeline as of now.
Crisis on Infinite Earths
Identity Crisis
Countdown to Infinite Crisis
The OMAC Project (with the Superman/Wonder Woman Sacrifice Crossover)
Villains United
Days of Vengeance
The Rann-Thanagar War
Infinite Crisis (with the Infinite Crisis one-shots that wrap up the four miniseries)
52 (with World War Three)
One Year Later
and now Countdown
And then whatever DC is now counting down to
Basically in Infinite Crisis, Alexander Luthor Jr. from the pre-crisis Earth 3 recreated the Multiverse in an attempt to create a more perfect Earth. He succeeded in a way, and created what they are calling "New Earth" which is mainline continuity (changing little things that had been conceived in the last decade... Wonder Woman was a founding member of the Justice League, Batman's parents were killed by Joe Chill rather than a nameless assailent, Superman now comes from the Superman: Birthright Krypton). Alexander Luthor started destroying an infinite number of worlds to try to alter them and change them... He was stopped in Infinite Crisis, and 52 identical earths were left in the new DC Multiverse. Mr. Mind, the real bad guy of 52 then went into the cosmic ether and basically was devouring the timelines of these multiple earths, changing them slightly or drastically from the New Earth continuity. He was stopped before he destroyed them all and we are now left with something similar, but not TOO similar to the Pre-Crisis on Infinite Earths Multiverse. Each Universe has a Monitor, and they will play a large part in the year to come.
The other thing that might be good to read is Winnick's run on Batman that reintroduced Jason Todd as the Red Hood...
An Idea for a Post Countdown, and post-whatever-comes-after-countdown weekly series: 52 Worlds - Each week a writer and an artist portray life each of the different worlds in the Multiverse... but I bet they want to keep it more open than that.