Twitter (www.twitter.com) is a micro-blogging service created by Jack Dorsey in 2006. Essentially it's like an SMS service on the internet. Users can post 140 character long messages, known as Tweets and friends, known as followers can read them.
Twitter is a completely free service and only takes a few minutes to sign-up for an account.
The service can be used in many ways which could be catagorised into: news, spam, self promotion, pointless babble, conversational and pass along value posts. Here is a selection of high profile individuals you can follow: Richard Branson, Simon Pegg, Stephen Fry or have a search for someone you are interested by.