On Saturday June 23rd, Microsoft adCenter did an upgrade which expanded negative keyword management to the campaign level. This will make campaign management easier for those with numerous ad groups, as negative keywords added at the campaign level apply to all ad groups in the campaign.
Adding ‘negative keywords' to either your ad groups or your campaigns keeps your ads from being displayed to customers who are unlikely to click your ad – or conversely, to the click-happy crowd.
For example, because I want to attract singles who are willing to pay for dating site memberships to Sage-Hearts.com, I like to add the word ‘free' to my list of negative keywords, so those searching the web for ‘free dating sites' won't see or click on my ads.
BTW, Microsoft adCenter's new account promo is still in effect. Get started for only $5 and get a $50 credit for ad clicks.