Twitter is a service that allows users to update what they are doing, exchange messages, etc. -- using a maximum of only 140 characters at a time. Individual messages are called tweets and are sent through text messages, instant-message services or the Internet. (See this Dana Milbank column in the Washington Post about senators and House members Twittering during the president's speech earlier this week.)

This article by the New York Times' personal technology writer explains the concept pretty well.

Here's the OCF Twitter feed. You can sign up to "follow" the tweets.