If you are managing PPC campaigns (including Google Adwords), you need to understand the importance and the cost saving power adding negative keywords to your campaign can give you.
In your campaign you can include keywords you don't want to show up for. And they can be included as a broad, phrase or exact match. And you can do it on a campaign level or an ad group level. Learn more about negative keywords here: Negative Match.
A technique I personally used for years to identify keywords I wanted a negative match for was to create redirects on my performing keywords. And I'd collect the 'exact' keywords people were actually searching on. The reason you had to do that is because, even if you are using conversion tracking etc., you weren't told what was being added to a phrase or broad match.
So you could be bidding on 'dog bones', but based on Google's system you'd show up for 'ancient dog bones' or 'dog bone history'. Your intent may have been to just show up for those searching for and wanting to buy dog bones for their dog to actually chew on.
The redirect script I used allowed me to collect that data and create reports that revealed everything and displayed it for me neatly. Then I could turn around and add those to my campaigns as negatives.
But today with Google, we don't need to rely on a 3rd party for this!
Today you can do this right in the Google interface using the "Search Terms" report.

Very slick!
YOU MUST! ...do this with all your phrase and broad match terms. Look at them on a regular basis. If you don't, then you are just handing over free money to Google and it can mean the difference between success and failure; profit and loss.
Through this effort you WILL find negative keywords and phrases that either have nothing to do with what you are selling OR simply don't convert well. Don't just throw your money to the wind -- target, target, target your keywords and block irrelevant ones!
-Matt Levenhagen
PPC Advertising - Keywords