I operate primarily in the tech field, so most of the people that I want to interview already know what a podcast is. However, there are tons of great interviewees out there who may have only heard the term in passing, or not at all.
I think the best way to describe a podcast to someone who has never heard the term before is to tie it to what they know. I'd start with saying that a podcast is essentially Internet radio. It has the added advantage, though, of being available online potentially foreve so that listeners can grab it at any time. A podcast can deliver a message forever rather than a one-shot radio deal or presentation.
Personally, I wouldn't bother with any explanation about RSS feeds and podcatchers as that's likely to make the whole thing sound too technical and may scare off your potential interviewee.
