I was talking to a friend about being at a concert while programming. That thoroughly confused them. I needed something to show them for a quick explanation. Trying to explain Second Life™ with words is pointless, in my experience.
The Lab has a good web page up that serves the purpose of explaining SL. While it falls short, the videos there are far better than my words and verbal efforts. I can just say, “Second Life is a second fun only life.” Send them to the web page and they seem to get a closer idea of what SL is. But, I have yet to see any answer to the question “What is Second Life?” that actually works for everyone.
Check out: What is Second Life?