It's basically a micro-blog. You can post to it from your cellphone or whatever and it updates on your site to let people know all the interesting things you're doing at that EXACT MOMENT. Yes, you too can post exciting Twitter entries such as:
"I'm eating a roast beef sandwich. It's alright."
"I think some urine just dripped onto my pants when I shook my junk."
"Shit, my cellphone battery is about to die!"
Yeah, it clear to me that this is a VITAL plug-in for any web site.
TWITTER - COMING SOON TO I-MOCKERY!