Want to know what Sen. Barack Obama is up to? If you keep track of his Twitter, you know that he “[t]alked with Communications Workers of America about getting America back to work” and you can follow a link to see his full speech.

What is Twitter, you ask?

Twitter is a service for friends, family, and co–workers to communicate and stay connected through the exchange of quick, frequent answers to one simple question: What are you doing?

It’s a hybrid of blogging, texting and instant messaging - or micro-blogging, as it has been called. Twitter users can send updates, called “tweets”, to their Twitter page in the form of text-based posts that are limited to 140 characters. The service is free at Twitter.com. Users can update on their computers or register a mobile device to send updates via text message. Friends can sign up to “follow” a Twitter, meaning that they get get a text message update every time their friend “tweets.”

What’s the big deal? Presidential candidates - or more likely, their campaign workers - are using Twitter to keep supporters updated on speeches, appearances, fundraising efforts, you name it. And people are following them.

Barack Obama’s Twitter currently has 22,063 followers. Hillary Clinton’s Twitter is trailing by a long shot with 2,823 followers. Perhaps it has something to do with Obama’s large youth base, which seems a lot more likely to embrace this new kind of technology.

It looks like presumptive Republican nominee John McCain has not yet jumped on the Twitter bandwagon. In fact, Twitter has only caused him problems - see “McCain Aide Suspended Over Twitter” from the TIME blog.

For more Twitter fun, check out John Edwards’s abandoned Twitter (4,332 followers) and Ron Paul’s nearly up-to-date Twitter (1,202 followers).