Many sites / blogs already have buttons that allow visitors to tweet their content, and those buttons are typically from Tweetmeme, third-party unofficial provider of Twitter sharing buttons.
Now, Twitter have launched "Tweet Button," embeddable on any web page, show comprehensive count of how many times the URL has been shared on Twitter. When using the button, the Twitter user never leaves the site where the button appears, according to the Twitter documentation.
The button come in three forms:
More Info: Tweet Button