I’m usually a pretty positive person, but there is one instance when it pays to think negative and that is pay-per-click campaigns. Pay-per-click or sponsored search systems let you specify negative keywords at the adgroup or campaign level.
Just add negative keywords or phases to your ad group or campaign means and ads won’t show for search queries containing that phrase. By filtering out unwanted impressions, negative keywords can help you reach the most appropriate prospects, reduce your cost-per-click (CPC), and increase your ROI.
An Example
If you are advertising on the phrase “computer networking” you probably don’t want your ads to appear for “computer networking classes” or “computer networking jobs” unless you: a) offer a class, or b) are hiring. Thus you could place “classes” and “jobs” in your negative keyword list and solve the problem.