The +1 button lets users recommend the content they like on the web, helping
to improve Google search for their friends and contacts. It appears on Google
search, Google search ads, and other sites across the web.When users click the +1 button for a particular webpage, their names and
profile pictures will appear as part of personal annotations in their friends
and contacts’ search results. Users will see their names and pictures beneath
the ads and organic links associated with the webpage. The final landing page’s
URL can also appear on the Google profile of any user who +1’s your ad or
organic result and chooses to share +1’s on her profile.Whenever possible, the +1 button is associated with a site’s final landing page. This means a user can recommend a webpage by clicking the +1 button on that page, or the +1 button next to a Google search ad or search result associated with that page. For example, if a user +1’s an organic search result with the same final landing page as your ad, the +1 count associated with both the organic search result and the search ad will increase.
Reference from: https://adwords.google.com/support/aw/bin/answer.py?hl=en&answer=187588
More info please see here: https://adwords.google.com/support/aw/bin/topic.py?hl=en&topic=1152956
How to use please see here: http://www.google.com/intl/en/webmasters/+1/button/index.html
Additional info please see here: http://www.google.com/intl/zh-TW/+/learnmore/