I guess there are a lot of ways to describe the concept of podcasting to beginners*, but the easiest I found, explaining to a few of my friends is basically this:
Podcasts are blogs for jogs, or put it another way, podcasters are bloggers for joggers. Most joggers that you might find in the park these days are likely to carry MP3 players, particularly and most popularly iPods. Now most bloggers these days, think of text blogging (this type) as a relatively old medium, so they’re switching to audio blogging, which in somewhat other words is podcasting. Now when you mix the joggers, and the bloggers, you get the concept for podcasting — which is, blogs you can carry in your iPod for jogs.
* For a more detailed explaination, e.g. how you actually do podcasting or listen to it on your jog, I would strongly suggest you try this query.