Beginning April 1, Google will no longer make exceptions to its policy requiring advertisers to match the display URL in an AdWords ad to the landing page to which it leads. Google's existing policy already requires that an ad's display URL matches its destination URL in the AdWords interface and the landing page to which it leads, but exceptions had been made for things like redirects or vanity URLs.
Under the new rules, all advertisers, regardless of past exceptions, will need to show users the same top-level domain in the display URL and the landing page where a user is sent.
The only exceptions allowed will be for using tracking URLs as the destination URL, as long as the URL of the landing page matches that of the display URL. For example, a display URL of www.google.com/adwords could use a destination URL in AdWords of www.trackingurl.com/google123, if the landing page where the user arrives is www.google.com and not www.trackingurl.com.
Display URLs will be allowed to show different subdomains or directories, as long as the top-level domain shown in the ad (such as google.com in the above example) is matched.