Twitter isan SMS-based service where you can update a group of people with asingle text message or a short message from the web. The message isthen sent to your buddies over SMS and is available to be queried overthe web. People can "subscribe" your presence and you can "subscribe"to other people's presence. You receive the updates over SMS.
I'm sure the carriers absolutely love Twitter because it drives theuse of text messaging. Too bad more people I know don't use it.