Twitter is a microblogging community. In other words, imagine sending out a text message to 2 people. Or 200 people. or 2,000 people. Now imagine being able to follow the text messages of the same amount of people as well. All these people interact with one another in a real-time environment. You get to see everything that is happening in the world first-hand, free of the bias and filtering of traditional channels. That is Twitter.

For more - Check out this slideshow for Twitter for Beginners from Paul Bradshaw.
Who is using Twitter?

As it emerged, twitter became home to technologists and first-adopters eager to use the latest and greatest in web 2.0 technology. As is such, a lot of twittterers are quasi-geeks like me or New Media Douchebags seeking a medium to market their goods! But as Twitter has gained traction, users beyond the technology sphere have begun to adopt twitter as well.