The Retweet Button was created by Tweetmeme, a service that aggregates links on Twitter to find popular topics sweeping through the microblog. Tweetmeme has a Wordpress plug-in to make it simple to include the button in any blog post, but the embed code can be added to any site.
Once a user clicks on the button, they're automatically redirected to their Twitter profile. A pre-written tweet is shown in their status update window saying, "RT @tweetmeme" followed by the title of the article and its link. After they update their stream, the Retweet counter adds one retweet to the tally.
There's a viral element to the Retweet button. A single user who retweets an article can drive thousands of people to the story, especially if the person's followers retweet as well. "Pass-along" traffic from retweets is becoming an important page view generator.
If you want to embed the Retweet button into your own posts, click here.