Twitter is a social networking and micro blogging site that allows users to send and receive updates (called tweets) about what they are up to. You can restrict your tweets to a circle of friends by following and being followed. More info can be found here http://en.wikipedia.org/wiki/Twitter.
I am now twittering! My username is paulgrimley (original) and you can find me at http://www.twitter.com/paulgrimley