Interesting thread.
Strangely, I never tried to explain PT and I very rarely mention it. My best friends (not all are pet lovers) and family don't know about it.

Once, I mentioned PT to the people at the shelter where I help and I only said it was a discussion forum about animals, that you could post pics of your furry kids and tell about almost everything else you want.
Although they are great cat lovers and they spend time, money, holiday time, etc...to rescue cats, they are not Internet people, and they also don't speak English well enough to easily chat. When I saw they were not really interested I did not insist.

If someone would have told me about PT, I have the feeling I would not have understood or I would not have been interested as well. Perhaps the thing is that you have to discover it by yourself...