If you don't know about Twitter yet, Wikipedia gives a nice overview:
Twitter is a free social networking and micro-blogging service that allows users to send "updates" (or "tweets"; text-based posts, up to 140 characters long) to the Twitter website, via short message service, instant messaging, or a third-party application such as Twitterrific.
Updates are displayed on the user's profile page and instantly delivered to other users who have signed up to receive them. The sender can restrict delivery to those in his or her circle of friends (delivery to everyone is the default). Users can receive updates via the Twitter website, instant messaging, SMS, RSS, email or through an application. For SMS, four gateway numbers are currently available: short codes for the USA, Canada, and India, as well as a UK number for international use. Several third parties offer posting and receiving updates via email.