If you were born before the 80's (heck maybe before the 90's) you have some idea what twitter is but can't understand what it is for. Well, I've been thinking about twitter and I have a few ideas.
A tweet (a message on twitter) is a short (140 char max) thought you broadcast to the world. Now the 'magic' of twitter is this: people can decide to listen to your message, only if they want to. If you think a tweet is stupid, it is because it isn't written for you, instead of complaining about the tweet, you ignore it '.' This should sounds familiar, it is how broadcast works in ethernet networks, and even how conversations work on public transit.
Since tweets are public, and arguably represent what people are thinking/doing, you can do something you could never do before: see what the world is thinking. From that starting point, you can join a conversation, or simply learn something interesting.
That said, as I'm looking at twitter, and I'm not finding any conversations I'm interested in joining, or even overhearing. Oh well I'll keep my eye on it.