Before launching a negative keyword campaign, let’s make sure everyone understands what it is. First of all, here’s a little background information on keywords. Keywords are words that search engines use to search for relevant Web pages.Negative keywords are used in keyword advertising to hinder ads from being shown when a search term contains the negative keywords.As Google clarifies, negative keywords aren’t for everyone. People are accustomed to using keywords to drive traffic to their site, but as everyone knows, not all traffic is good traffic. This is where negative keywords come into the game.If you are a retail company and want specific, targeted traffic, a negative keyword ad campaign is something you might want to consider. For instance, if you’re a company that only sells women’s clothing, you could put “men’s clothing” as a negative keyword.Each of the search engines are different when it comes to their allotment of negative keywords. Google allows a maximum of 10,000 terms or phrases. Yahoo! allows 250 maximum terms or phrases. The total character count for all negative keywords for Microsoft cannot surpass 1,022 and no single keyword can go beyond 100 characters.Being very specific and targeted is key to a successful negative keyword ad campaign. There are several tools available to help you in this effort. Google offers several tools such as Google Keyword Tool, Google Suggest, and Google Product Search. WordTracker is another tool that helps you to filter that unwanted traffic. Ken Jurina (as seen in the above WPN video from PubCon 2008) and the Epiar crew recently announced a new negative keyword service that uses their proprietary keyword research application to generate a prioritized negative keyword list.