Taken from Wikipedia at http://en.wikipedia.org/wiki/Twitter.
"Twitter is a social networking and microblogging service that enables its users to send and read messages known as tweets. Tweets are text-based posts of up to 140 characters displayed on the author's profile page and delivered to the author's subscribers who are known as followers. Senders can restrict delivery to those in their circle of friends or, by default, allow open access. Since late 2009, users can follow lists of authors instead of following individual authors. All users can send and receive tweets via the Twitter website, Short Message Service (SMS), or external applications (notably including those developed for smartphones). While the service itself costs nothing to use, accessing it through SMS may incur phone service provider fees. The website currently has more than 100 million users worldwide."
Wikipedia's article seemed to say a bit more than Twitter's site about the company. Visit Twitter at http://twitter.com/. Let me know if you think this might be of interest to you in the library or classroom. I have my doubts, depending on the situation of course!