The Google +1 button lets people who love your content recommend it on Google search, and helps people discover relevant content—such as your website—from the people they already know and trust. The +1 button appears on Google search, on websites, and on ads. For example, you might see a +1 button for a Google search result, Google ad, or next to an article you're reading on your favorite news site.
Adding the +1 button to pages on your site lets visitors recommend your content, knowing that their friends and contacts will see their recommendation when it’s most relevant—in the context of Google search results. Content recommended by friends and family is generally more relevant than content from strangers. For example, your friends may have the same taste in music as you do, have their finger on the social pulse, or know where the best shopping bargains are to be had - and so you'll visit websites that they +1.
For more information about +1, and how it improves your SEO, visit: http://www.google.com/webmasters/+1/button/.
Here's how to add the Google +1 button to your site: