It's the new microblogging craze. Basically posting 140 word posts for people to read. It's a decent jump-off for getting articles and short bits out there quickly (like, say "rumor: Doron Lamb commits!") instead of making a whole blog post or news article. It's social networking, and easily accessible on your mobile phone, which makes it more attractive than trying to read a whole blog post on you celly.
(
http://en.wikipedia.org/wiki/Twitter)
But it also strips the content + nuance right out of what you're trying to say/ broadcast. It'll probably be replaced by something in 2-3 years, like facebook or most social networking sites. But right now, lots of cats are on it - writers, head coaches (Calipari's on it:
http://twitter.com/UKCoachCalipari, among others), players (some player got in trouble for sending a message during halftime of an NBA game, I think it was Dwight Howard).... So it's kind of the hot thing now.