What you see on the right is a 300x250 rectangular Google AdSense Ad unit. Ideally an ad unit this size should show 3-4 different ads but maybe there isn’t enough inventory and therefore, all you see is 1 ad somewhere near the middle of the rectangle.
When a website visitor hovers the mouse over this Google Ad block, the cursor changes from a pointer to an hand and he can click anywhere inside the rectangle to visit the advertiser’s website.
However, this will soon change as Google has redefined the definition of a click.
Now a visitor will have to click either the Title or the URL of the Google Ad for the publisher (you) to make money.
Going back to the screenshot, only clicks made in the Red Area will be treated as valid while the Yellow Areas will not be clickable. In the current AdSense Ad formats, the entire block (both red and yellow) is clickable.
A site visitor who is genuinely interested in the advertiser’s site will have to position his mouse cursor exactly over the red area in order to visit the external website. This could reduce the overall clickthrough rate and you may see less earnings but it’s definitely a good thing in the long run both for publishers and advertisers.
Google says that similar functionality exists for ads appearing on the Google.com main website but there’s a small difference - only the title is click on Google homepage but for AdSense ads on the publisher’s website, both title and advertiser’s website URL will be clickable.
From AdSenseAdvisor - “To clarify some questions that have come up, this means that only clicks on the title or URL of a text ad will take users away from your page, register as clicks in your account, and result in advertiser charges. Clicking an ad’s whitespace will no longer do anything. We’ll have more details about this change in an AdSense blog post coming soon.”