During the past two weeks, the Bing team has been hard at work rolling out new features, which followed the adCenter release of several new features including search query reports. As we head into the holidays, it is important to understand the differences between natural search results compared to sponsored sites (pay-per-click ads) and where they appear on the search engine results pages (SERPs).
http://community.microsoftadvertising.com/blogs/advertiser/archive/2009/11/25/beginner-s-guide-to-sem-natural-versus-paid-search.aspx