What are you doing?" It's an innocent question that's the fuel for Twitter, one of the web's fastest growing sites. Never heard of it, you say? You've obviously never been tweeted.

The site started as a social-network service that lets you tell people what you are up to at any given moment and it's been rapidly gaining in popularity (traffic has risen eight-fold in the past year). Twitterers "tweet" via mobile or on the web about everything from what they had for lunch to how much they enjoyed their latest DVD. You can send a 140-characters tweet to the site, which will appear online and be sent directly to those who have chosen to follow you. If that sounds silly at first, don't worry, you're not alone.

"When people hear about Twitter, their immediate reaction is that it's the simplest and stupidest idea in the world," says the co-founder Biz Stone. "They do not want to know what their brother is eating right now. But then they discover that their friends are on it. Then they get it. Twitter took off really quickly, and honestly, we were surprised and had to play a lot of catch-up."

Twitterers, as they call themselves, post their updates at twitter.com or by using text. A cottage industry of web sites - including TweetScan, FriendFeed and Summize (which Twitter recently acquired and renamed Twitter Search) - have popped up to service the Twitterers and their tweets, by making it easier to search through the chatter for specific topics or people. Many companies (including the BBC and CNN) have responded to the popularity of Twitter by offering instant news updates to share with friends.

To get started on Twitter, you begin by searching to see who else is using the service and ask permission to "follow" their postings. Twitter subscriber Joe Rogel says the service is a great way to reach those who might be otherwise inaccessible.

Not everyone loves Twitter. Phil Leigh, an analyst for Inside Digital Media, says he goes on the site with an open mind and just doesn't get it. "That some guy saw Wall-E and thought it was a great movie is wonderful, but it's just not that interesting to me. If somebody has something important to say, they can say it in an e-mail."

Twitter's problem is keeping its users happy. So many people go on it that at times - often, in fact - the system crashes, and Twitter is unusable. Stone and co-founder Jack Dorsey say the problem is that Twitter is so popular and that the system they created wasn't built for masses. An influx of engineers is working to rebuild it, and they say the situation should be resolved within the year.

Twitter's success is realising that people don't want to use the web for private conversations but public ones. Nearly 90% of Twitter users make their updates public, so everyone can read them.

"It encourages other people to see what they're saying," says Stone. "People aren't doing one-to-one e-mail or instant messages anymore. Just look at comments on MySpace and blogs. They're communicating with one another in an open way."

In choosing a name for the service, Stone suggested Twitter, and the co-founders jumped for it. "It's what birds do when they converge," says Stone. "The sound they make is technically defined as a trivial chirp. During events, you can move as one with your friends, just like birds, because you all know what everyone is up to."

And if the bird analogy doesn't persuade you to use Twitter, we'll leave the last word to Dorsey: "Is there anyone you care about? Twitter is about keeping in touch and making the world smaller."