For years, it has been accepted that all of the heroes owned by DC Comics live in the same universe. Superman, Batman, Wonder Woman, etc., all live in cities not too far from one another. They started a club to fight crime together (The Justice League of America). They occasionally trade villains (The Joker has been to Metropolis; Lex Luthor helped rebuild Gotham after it was hit by a massive earthquake). For the most part, they have maintained a cohesive universe, but after 70 years, some inconsistencies are bound to show up. Comic book fans love to point these things out ("How can Batman be helping to rebuild Gotham after if he just got sent to the 853rd Century by his futuristic counterpart? And why isn't Superman helping with the rebuilding? Fighting an army of robotic duplicates of himself can't be THAT hard!").
Over the years, when the inconsistencies got too far out of hand, DC comics would do some massive housecleaning. In 1985, they released a series called Crisis on Infinite Earths. In this landmark series, the very fabric of time and space was threatened, numerous alternate realities were merged into a single universe, and many heroes and villains were killed along the way. Now, 20 years later, to deal with the inconsistencies that were left in the wake of this series, they are about to release another series this fall called Infinite Crisis, which will reboot the DC Universe yet again.
They have been dropping hints in various comics over the past year or two as to what is going to happen. Prelude to Infinite Crisis compiles scenes from these comics, and reprints some key stories which will feed into this upcoming series. It's also an excellent primer to the DC Universe as a whole. And it's $5.99, which, at 96 pages, is a bargain compared to a lot of the comics out there.
In addition to Prelude to Infinite Crisis, DC's website is offering Crisis Counseling. Every Monday, they will update readers on events that happened in the previous week's comics which will lead into Infinite Crisis. I think it's an excellent way to ease new readers into the fold, and well worth a look.
Recommended for: people who want an entry point into the DC Universe; people who like to get in on the ground floor of a big event.
Not Recommended for: people who like smaller, quieter, more realistic stories.