From Twitter's blog:
Today we’re launching the Tweet Button to make sharing simple. It lets you share links directly from the page you’re on. When you click on the Tweet Button, a Tweet box will appear—pre-populated with a shortened link that points to the item that you’re sharing.
The Tweet Button is not only simple for users, but for publishers of all sizes, too. Recreational bloggers to large media companies can quickly and easily add the Tweet Button to their sites. It only takes a few lines of code. The Tweet Button will help publishers grow traffic and increase their Twitter following.
Twitter does acknowledge third party apps that created this service, blogging to users, "you may have seen similar buttons on blogs, news sites and other places that let you share content on Twitter. These have been created by third parties. Most notably, a company called TweetMeme created a popular 'retweet button' for publishers." Twitter has stated they're working closely with TweetMeme, who will close its service in favour of Twitter's official button. TweetMeme is now onto a new project called DataShift. Tweetmeme generated 750 million impressions per day according to the company's founder.