"The keywords you choose for content ads play a different role than they do for search ads; a point that's often tough for search advertisers to grasp. In traditional search advertising, keywords are intended to match search queries that are usually related to the advertiser's products and services. The keywords in a content ad group should describe the types of pages and sites where you want your ads to appear.

The number one reason for content advertising campaign failure is because ads frequently appear on untargeted, irrelevant pages, but you can control ad placement more precisely by including a small number (20-40) of keywords in your keyword-targeted content ad groups."

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http://searchenginewatch.com/showPage.html?page=3628949