Twitter is a multi-platform service that allows users to send "updates" (text-based posts, up to 140 characters long) via SMS (text message), instant messaging, and email. In its most common application, people send Twitter messages via SMS telling what they are doing at that very moment ("eating breakfast," "lots of traffic this morning," "lunch break").  Then people who have subscribed to that feed can then read these messages as they are sent out, allowing them to know what their friend is doing.

The blog Vancouver Social Enterprise Forum has an interesting post about how activists are using Twitter as a