The +1 button is Google’s answer to the Facebook “Like,” and the Twitter “Favorite.” Essentially, the +1 button now appears across the web, both on Google search engine results pages (SERPS), and individual websites themselves. The point of the +1 button is twofold: (1) it let’s you publicly endorse a webpage, and (2) it allows Google to improve the quality of its search engine results by incorporating your “endorsements” into its future results.
Essentially, whenever you see a +1 button, you should understand that Google is asking you for your opinion on whether it is a good, bad, or unremarkable site. You should know that, although you can control your privacy settings, and you can control who sees what you’ve +1’d online, you need to have a public profile with your real name (according to Google’s terms of service) in order to use the +1, and so there are privacy implications around this topic.
The +1 button is a feature of the Google+ social network.