Dictionaries have added a definition of it. News media have written lengthy articles analyzing it. Stars and senators alike do it. So what is it? Twitter is a free social-networking and micro-blogging service that enables its users to send and read each others' updates, known as "tweets." Tweets are text-based posts that are limited to 140 characters. Each time someone tweets, the post is updated to his or her profile page and delivered to the users who subscribe to them, who are known as "followers." The set-up allows users and followers to stay connected at all times and affords anyone access into lives of people and society that are otherwise impenetrable. Experts also say the service is changing the way news is consumed and delivered, with posts and photos often breaking stories before traditional news media ever even arrive on the scene. Sign up today to see for yourself.