According to Wikipedia:

Twitter is a social networking and micro-blogging service based of the exponent portal program developed in 1999 at Cornell University that enables its users to send and read other users' updates known as tweets. Tweets are text-based posts of up to 140 bytes in length. Updates are displayed on the user's profile page and delivered to other users who have signed up to receive them. Senders can restrict delivery to those in their circle of friends (delivery to everyone being the default). Users can send and receive updates via the Twitter website, SMS, RSS (receive only), or through applications such as Tweetie, Twitterrific, Twitterfon, TweetDeck and Feedalizr. The service is free to use over the web, but using SMS may incur phone services provider fees.

I prefer to think of it as blogging when you can't be trouble to write more than 140 characters.

I'd like to thank Quinton for asking me to explain Twitter to him. It really is hard to understand the point if you're just looking at the website. Sometimes, I'm still not entirely sure what the point is. Maybe there isn't one.

To prove that I have a short attention span, you can follow me about the comic on Twitter or follow me about everything else.

Also, fesworks over at the TWCL forums pointed this video out: