It's basically a site dedicated to status updates. Lily Allen is currently using it to take aim at News of the World and Perez Hilton.
Oh, and im on it too...Twitter is a social networking and micro-blogging service that allows its users to send and read other users' updates (known as tweets), which are text-based posts of up to 140 characters in length.
Updates are displayed on the user's profile page and delivered to other users who have signed up to receive them. Senders can restrict delivery to those in their circle of friends (delivery to everyone being the default). Users can receive updates via the Twitter website, SMS, RSS, or through applications such as TwitterMobile, Tweetie, Twinkle, Twitterrific, Feedalizr, Facebook, and Twidget, a widget application. Four gateway numbers are currently available for SMS: short codes for the United States, Canada, and India, and a United Kingdom-based number for international use. Several third parties offer posting and receiving updates via email. Estimates of the number of daily users vary as the company does not release the number of active accounts. In November 2008, Jeremiah Owyang of Forrester Research estimated that Twitter had 4-5 million users. A Bloomberg article in February 2009 stated the number to be 55 million daily users.