I've read a bunch of different ways in which Google Wave has been explained, but I have had the opportunity to actually demo and use Wave in the real world. I've introduced personally face to face at least a dozen new users to Wave in the last two weeks -- here is what I've learned.
People love Facebook. People hate that everyone can see everything on Facebook. People want their legal information as available as Facebook and as private as email. Explain Wave as Facebook for only those people you want involved -- that simple -- and almost everyone gets it. (Oh and throw in that it has the benefit of being real time).