You know I'm a fan of the folks at Common Craft, who have the unique ability to translate the strange world of new media into words that just-plain-folks can understand. They have a whole series of "...in plain English" videos, some of which I've written about here.
While writing a description of an upcoming Twitter session that I'm moderating for the Enterprise 2.0 conference here in Boston June 22-25, I found this one, which I'm posting for everyone who's asked me about Twitter, Web 2.0, or anything else that wasn't a word, phrase, or idea, say, 20 minutes ago.
Be advised: this explanation focuses on the use of Twitter with friends. There are countless other uses - from emergency work to news to marketing to customer service to...you name it. Keep it short and it Twitters just fine.