Twitter is an online service that lets you broadcast short messages to your friends or "followers." It also lets you specify which Twitter users you want to follow so you can read their messages in one place.
Twitter is designed to work on a mobile phone as well as on a computer. All Twitter messages are limited to 140 characters, so each message can be sent as a single SMS alert. You can't say much in 140 characters. That is part of Twitter's charm.
Since launching in 2007, Twitter has seen tremendous growth. In early 2009, Ashton Kutcher became the first user to gain more than a million followers. Oprah Winfrey then gave the microblog a plug and caught up to the million follower mark within a month's time. In late May, the tool also received its first use from space, courtesy of U.S. astronaut Mike Massimino. The tool was also an instrumental PR tool for presidential candidates in the 2008 U.S. election.