Microsoft announced the beta release of PubCenter at the AdSpace Conference this week. PubCenter is a solution designed to directly compete with contextual (i.e. matching the content of the page) affiliate/publisher programs like Google AdSense and the Yahoo Publisher Network (YPN).
As with all other contextual programs, webmasters display advertising coming from an advertising network - in this case the Microsoft ad network. Those displaying PubCenter ads receive compensation when ads are clicked or for the number of gross impressions the ads generate on the publisher's site.
Affiliates can sign up for the beta service now, but it's only available to publishers in the U.S. currently. Not that I have anything against Adsense, but as an affiliate you have an obligation to pursue the highest payouts and the best experience for users. As such, it is important for publishers to test multiple content advertising solutions and of course varying configurations of those solutions in order to get the maximum returns.