Evan Williams revolutionized blogging in 1999 by making it simple. His software, Blogger, asked for only one thing: text. It was easy for anyone, and blogging quickly became an activity more popular than many had believed possible.
Other blogging software adds more and more features, and even Blogger allows for optional titles, but Twitter takes Blogger's single-required-field simplicity and takes it down a notch. No titles, and now with a 140 character limit.
Jack Dorsey started in 2000 with an idea for tracking "status," a feature of the LiveJournal software he was using at the time. Over the next six years he refined and polished the idea, and finally wrote it with Biz Stone and a contract programmer in two weeks in March 2006. The concept moved from stat.us to twttr.com to twitter.com. Twitter asks just one question: What are you doing?
You can answer that question at twitter.com, or at m.twitter.com on your mobile device, or via instant message (GTalk, Jabber, or LiveJournal for now), or by sending an SMS from your phone. Because there is an open API, dozens of third-party applications are available, too.
SMS, by the way, is the reason for the 140 character limit. SMS allows 160 characters, but Twitter subtracts 20 for a username. Twitter has shortcodes in the U.S., India, and Canada, and they're very enthusiastic about SMS especially.
Less Than Public, More Than Private
Instant messenger is one-on-one communication, while a blog is available for all to see. Twitter occupies a place somewhere in between. While tweets can be “protected,” they are generally available for everyone to see, but only those who choose to “follow” you are likely to see them. Tweets can still be directed to individual users, creating lengthy conversations, and followers of one twitterer but not the other will see only half of the conversation, eventually likely having their curiosity stirred.
Not Enough For Some
There are competing services, most of which seem dedicated to the idea that Twitter would be much better if it just had... whatever. Just as many Blogger competitors found the sweet spot for a certain group of users by adding features, so too will some of Twitter's competitors find that Twitter+images or Twitter+podcasts or Twitter+video works well for them. Of course, Twitter's API also makes it easy for competitors to become partners instead, delivering images or podcasts or video using Twitter in conjunction with their own website or software. In an interview with BC Magazine, Twitter co-founder Biz Stone stated that the Twitter API serves 10-20 times more traffic than the Twitter.com website.