Initially, Google's answer to help small publishers do direct ad sales while filling those ad spots with AdSense when the direct ads were not available, was the DoubleClick for Publishers (initially called Google Ad Manager). Truth is, using it is complex for most publishers.
So Google AdSense came out with a new method yesterday named AdSense Direct, which basically swaps out your AdSense units with your direct sales and then when that campaign ends, the AdSense units return. What does Google get?
(1) More AdSense placements when your direct ads are not live where they get about 30% of the revenue. (2) On the direct ads, Google gets to 15% of the ad revenue. (3) Advertisers need to use Google Wallet.
But for many small publishers, this is a no brainer. Direct Ad sales can bring in more money but are hard to get. So when you get one, using this platform might make more sense.
Here is a video on how it works:
For more details, see the FAQs.
It seems like many publishers are not terribly into the idea of letting Google or AdSense manage their direct ad sales. A WebmasterWorld thread has senior member Netmeg saying "Not sure I'm on board but I have to look into it more."