AVC.com always has great posts.
In this one, "Understanding Twitter," the comments show enormous support for a tool that already seems like it's always been around, even though it's still new.
Twitter is so many things, and they all start with tweets:
-- Community: crises, disasters
-- Promotion: events, products, brands
-- News and information: #ongoing #topics
-- Personal AP wire: people tweeting to their communities as editors/curators
-- Personal/public: friends @ friends
I have just about abandoned my Facebook account and use Twitter in pretty much all these ways.