JenSense was the first I have seen to jump on the news that YPN added ad category targeting. What does Yahoo! Publisher Network Ad Targeting do? Simple, it allows you to define on a page, directory or site level the category of ads you want displayed. For example, on this individual archive page, if you see a YPN ad on the left side (might see Google AdSense, so you can refresh to get YPN), it should be about Computer Security, because I selected the Ad Category: Computing and the Ad Subcategory: Computer Security. I then associated this specific page with the ad target category. So even though this page is really about contextual ad networks and YPN, it should show computer security ads.
JenSense has more details and screen shots here but here is a screen shot of the "Manage Ad Categories" section with a page added.
Forum discussion at DigitalPoint Forums.