Overture is Yahoo's advertising network just like Adsense\Adwords is Google's.  All you have to do is type in the keywords your interested and search for them.  I will warn you that sometimes this tool is not available due to the amount of searches that people make and we never know when Yahoo will get rid of it.  When you get the search it provides a list of words that were searched and the amount of times searched keep in mind this is on a monthly basis.   Also remember that the Overture Tool combines the result for plural and singular searches.  For example television and televisions will be combined togethere for the number of searches.

 Everyone has a different threshold when they think it's good to register a domain name.  Some people like to see at least 10,000 searches, but for some names less may still be worth it.  For instance investment keywords tend to pay better than others so you may not need as many searches to make money on the domain.  The best use of Overture is type in an actual domain name with extension like domainnames.com versus searching domain names and you get an idea of actual traffic to the domain.  A lot of people feel if they can get an OVT (Overture Tool) score of 100 with a domain name extension then it's worth registering otherwise they leave it alone, of course everyone does it a little differently.  Also hyphens don't seem to come back with accurate measurements with Overture Tool.

 Please remember this is only one tool don't rely entirely on it.