It's a kind of social networking site, and is generating huge amounts of buzz among the web's early adopters thanks to a simple conceit. All twitter.com does is ask: "What are you doing?"
Source: Guardian Unlimited Technology
I can't really see the attraction myself, but never one to pass comment on something I don't know anything about (grin); I've set myself up with an account and I'm going to try to keep it updated over the next few days (or until I get bored), to see if I can work out what all the fuss is about. If you guys want to play along, you can check it out at http://www.twitter.com/garyshort.