Send and receive messages to and from mobile phones (by SMS), webpages, or from IM/Jabber/gchat. Send messages to a group of 'followers' or send direct messages to one person.
To send out developments of a campaign or action to people's mobile phones.
For lengthy commmunication
If you are using Twitter through your mobile phone, it has local numbers for use in the USA, Canada and India. You can use Twitter through the web anywhere in the world where there is an internet connection.
You can 'protect your updates' and authorise 'followers' who are then allowed to see your messages.
Limited, like text, to 140 characters per message.
Its a lightweight mass-SMS sending system so it can be updated (and read) via mobile phone, and can reach people working in emergency situations in a way the internet cannot.
Alaa's Twitter feed (http://twitter.com/alaa) has used twitter to effectively to coordinate protests in Cairo.
Obvious, http://obvious.com/
They claim 'What's yours is yours'
There has been some concern over privacy and authentication methods on twitter. Caution is advised in putting sensitive information over twitter. See http://blog.spywareguide.com/2007/04/first_spoofing_attacks_against.html for more info.
