Whenever I mention Twitter to anyone who wasn’t previously aware of it, the first question they ask is always ‘What is it for?’
Like many social web services, it can be difficult to explain the concept of Twitter. I find it is best to respond with another question: ‘Well, what is talking for?’
‘Er, telling people about stuff. Asking questions. Finding out what’s going on.’
That’s what Twitter is for, too. It’s just like talking.