Google’s +1 button allows Google search users to “like” both individual search results and AdWords ads in the search results.

+1 shows up as an icon to the right of search results and ads (see illustration at right). If you are logged in to your Google account, the first time you click a +1 button you get a popup that confirms you want to +1 the result or ad.

Notice the check box marked “Use my information to personalize content and ads across the web.” If you leave this checked, Google will over time try to serve you ads and search results that line up with what you have +1ed in the past.  (Did I just coin a horrible new verb there?). Everything you’ve +1ed also shows up in your Google Profile in a brand new tab. In your profile, you can remove +1s that you clicked previously.

People who are opted in to +1 and who are in your Google social network (Gmail contacts, Buzz , Reader and now Google Plus friends) will see your avatar and name next to results and ads that you’ve +1ed, and you will see theirs. You can also share your +1ed items with others.

The Google +1 button is now available to site owners as well. This makes it very clear that +1 is Google’s attempt to push back at Facebook, especially the prevalence of Facebook’s Like button for sites. Tied in with the rapidly growing Google+ (Google Plus) social network, Google is poised to gain a rich diet of social cues for its search products.

UPDATE: According to a Search Engine Land interview with Google personell, paid search marketers testing +1 saw a significant lift in CTR for ads that had been “plussed.” Google is linking +1s between organic and paid ads, so if one is plussed by someone, the other linked to the same URL will be also.