To give this new form a little context, here are three different cuts at definition:

1. Twitter's description: A global community of friends and strangers answering one simple question: What are you doing? Answer on your phone, IM or right here on the web! In an April 22, 2007 New York Times interview, Twitter co-founder, Evan Williams, said: "Twitter is best understood as a highly flexible messaging system that swiftly routes messages, composed on a variety of devices, to the people who have elected to receive them in the medium the recipients prefer. It is a technology that encourages a new mode of communication…"
2. My description: Microblogging meets global social networking. All the ways you connect digitally -- instant message, email, SMS/mobile, web -- combined with your select network or populating globally with a 140 character-limit. The short form is a big part of the function. Twitter is an active stream and archive of brief thought bursts bound to expedite and influence taste, news, popular culture and perhaps even where you'll dine tonight or in the near future and with whom.
3. Wikipedia description: Twitter is a social networking service that allows members to inform each other about what they are doing and what they think. It allows users to send messages via phone, instant messaging or the Twitter website. Two SMS gateway numbers are available: one for USA and one UK number for international use. Users can receive updates from other selected users via web, IM or SMS. It made its debut in March 2006. It is an example of a microblogging platform.
   
   "Quoting the Wall Street Journal March 16, 2007 article, 'These [social-networking] services elicit mixed feelings in the technology-savvy people who have been their early adopters. Fans say they are a good way to keep in touch with busy friends. But some users are starting to feel 'too' connected, as they grapple with check-in messages at odd hours, higher cellphone bills and the need to tell acquaintances to stop announcing what they're having for dinner.'"

Note: A new service has since been launched that mitigates most of the "higher cellphone bill" concerns. EmailTwitter.com lets anyone post and retrieve Twitter updates by email without incurring SMS fees.