Still not sure what it is?
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 (e.g. on a cell phone), instant messaging, or a third-party application such as Twitterrific or Facebook.
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 (text messages), RSS, email or through an application.
*warning: Twitter use can be highly addictive and habit forming, resulting in the tendency to scrap all your Facebook, MySpace, and miscellaneous IM accounts… Use with discretion and caffeine.