Chances are you've visited at least one Google site or another today, and chances are you were delivered some ads tailored just for you while you were there. Exactly how those ads got from the advertiser to you hasn't always been clear, but Google's now announced a new effort that it promises will provide "greater transparency and choice" regarding the ads you see. The most immediate change is a new "Why this ad?" link that will be displayed in search results and on Gmail -- click on it and Google provides a brief explanation of how that particular ad came to appear on your screen (based on your current search terms, your location, etc.). You'll also soon be able to access a new Ads Preference Manager, which will let you block specific advertisers or opt out of personalized ads entirely. Google says those options will be appearing "over the coming weeks," but you can get a peek at what's in store in the video after the break.