This is a guest post by Shawn Livengood. Shawn Livengood is a search marketing professional based in Austin, Texas. He is also pursuing his master’s degree in information architecture at the University of Texas at Austin. You can read more at his PPC marketing blog , PPC Without Pity.
When you sign up for a pay-per-click advertising account, you might assume that you are buying your traffic directly from the search engine. If you bid on keywords in Google AdWords, you're just getting traffic from people who search on Google.com, right? Wrong! What many people don't realize is that a significant portion of PPC traffic comes not from the search engine sites themselves, but from a network of "search partners" associated with each search engine.
So what exactly is a "search partner"? Basically, it's any other site that uses Google or Yahoo search bars on its pages and displays PPC ads  from these networks in the search results. Bing (formerly MSN) doesn't currently have a similar program, but I wouldn't be surprised if they started one in the near future, considering Microsoft's big push over the past year to increase use of the Bing search engine.
Google's major search partners include Google properties (Google Maps, Google Groups, etc.) and major websites and networks such as Virgin Media, Amazon.com, and MySpace. The search partner network also includes every little mom-and-pop blog that uses Google's free site search feature in exchange for showing Google ads  on their site search pages. Yahoo has a less extensive partner network, but still includes such major presences as Info.com, Local.com, Embarq.com, and Verizon.net. Since Yahoo also offers site search solutions, you do stand a chance of having your ads show up on much smaller partner sites.
If you do opt in to these search partner networks, expect to get traffic of a much lower quality than what you get directly from the search engines. When someone searches on Google or Yahoo, they are in an active searching mode, and are likely to read, click, and purchase from PPC ads. If the user is on Bubba's Widget Blog (now with Google Site Search!) ... well, not so much. But don't worry! There is a way to escape from search partner purgatory.
Google has made it pretty easy to get reports on search partner traffic, and opt out if you want to. To get information on search partner performance, just run an Account, Campaign, or Ad Group report, open up "Add or Remove Columns" in the Advanced Settings, and check off the box labeled "Ad Distribution: with search partners" in the "Attributes" section:
If you don't like what you see in the report, you can always opt out of ad distribution on Google's search partner network. To do this, go to the campaign you want to opt out, and open up the "Settings" tab. Click "Edit" next to the "Networks" option:
In the subsequent menu, uncheck the box labeled "Search Partners," and your ads will no longer show anywhere except for direct Google searches.
Yahoo doesn't quite have this level of transparency yet, but they will be rolling out a similar feature sometime in January 2010. Once this feature goes live, you should be able to target your campaigns and ad groups to Yahoo Search, Yahoo Search Partners, or both. Let's hope that they offer good reporting on search partner performance as well.