The tool is called "Ad Interest Manager" and it lets you opt-in and out of ads based on interests and preferences. Here's a screenshot:
The Ad Interest Manager also shows you what Yahoo! activities you conduct the most. They use this to help determine which ads to serve up to you, personally.
Last but not least, the Ad Interest Manager displays information about your IP address, location, age range, and gender which are gained from your computer and cookies. Ads can also be served up to you based on that information. I'm not posting a screenshot of my info - you'll have to check that out for yourself.