Also like blogs, everyone has their own unique definition of what Twitter is (stripped down blogs, public IM, Dodgeball++, etc.), and to some extent, everyone is correct. Maybe that’s when you know how you’ve got a winner: when people use it like mad but can’t fully explain the appeal of it to others. See also: weblogs, Flickr.
The way I see it, Twitter is to IM and SMS what weblogs were to email and Usenet.