I thought this blog essay at Scientific American was as good a description of Twitter as anything I’d seen — Twitter Is a 24-Hour Party. You Can Tweet If You Want To.
Personally I’ve written that if you imagine the Internet as a big office, then Twitter is the conversation around the coffee pot in the break room. Go to Twitter when you want the gossip, or the sports news or the political chatter, or if you’re just restless and looking to be amused or informed. But in any case don’t ever think of Twitter as an inbox that has to be read.
The key of course is to find the most interesting people to have in the break room with you.