According to Twitter's homepage it is:
"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?" Twittering is known as a way of micro-blogging where you share tidbits of information about what you are doing as opposed to a full blog post where you can share much more information. Twitter posts are limited to 140 characters. Many people use it to post links to their blogs or to give quick-hitting updates of what they're up to at the moment.
A program aimed to help you tweet quicker and easier.