According to Wiki...
Twitter is a free social networking and micro-bloggin 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 Twitterific 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, text messages on their phone, email or through an application.
It's called micro-blogging. A great way to be connected with people around the country or in your own neighborhood. I've been doing it for a bit, like it and have connected with other people in my profession around the country. You can read my updates over there on the left or follow me http://twitter.com/johnincolorado