At certain events, I am introduced as the social media or insights maven. Thus, I often find myself in situations where I have to explain: "What is Twitter?". As I was on youtube, I found this cool video which could help me and help you explain what is Twitter in plain english.
http://www.youtube.com/watch?v=ddO9idmax0o

I find the easiest way to explain twitter is that it is another means of communication and interaction that limits you to a certain message length, forcing you to get to the point and cut through all the bullsh*t. I also find that equating it to texting helps people to understand it easier.