Twitter is a social networking and microblogging service, owned and operated by Twitter Inc., that enables its users to send and read other user messages called tweets. Tweets are text-based posts of up to 140 characters displayed on the author's profile page. Tweets are publicly visible by default, however senders can restrict message delivery to their friends list. Users may subscribe to other author tweets—this is known as following and subscribers are known as followers. As of late 2009, users can follow lists of authors instead of following individual authors. All users can send and receive tweets via the Twitter website, compatible external applications (such as, for smartphones), or by Short Message Service (SMS) available in certain countries. While the service is free, accessing it through SMS may incur phone service provider fees. The website is based out in San Bruno, California near San Francisco (where the website was first based). Twitter also has servers and offices in San Antonio, Texas and Boston, Massachusetts.
