“Negative keywords” are one of the most important concepts in Google AdWords advertising. These are the search words which you do not want triggering your ads. For example, if you sell blue widgets directly, you’re going to be after people who want to buy blue widgets, not people trying to find out how they work or where they can get a picture of one. So by all means advertise against any search which contains the term “blue widgets”, but then make your list of exceptions, such as searches containing the word “how” or the word “photo”.
If you’re thinking: “Well, my ad says ‘buy blue widgets here’ so nobody wondering how they work is going to click on the ad and cost me money”, you’re mistaken. Some people will click on anything. That’s why you don’t run your blue widgets ad against searches for “Lady Gaga”, despite the millions of views your ad might get every day.
If you manage your own campaign, you really do need to take a good look at your negative keyword lists, and if you have somebody else managing things for you, it’s worth checking that they have a decent (and ever-evolving) selection. All of our clients do. Take a look at the actual search terms which triggered your ads and start to find the words which feature in the searches where you don’t want your ad to show any more.