Think of your favorite radio or television show. Imagine that you automatically receive new episodes the very instant they are released (for free), and you can either listen to them right on your computer, or take them with you for consuming at your own convenience.
That’s a podcast.
The term “podcast” can be slightly misleading. There is no iPod inherently necessary to either create or listen to one, and there is no live “broadcast” taking place. A podcast is nothing more than: (1) a pre-recorded/produced show (whether it is audio or video) that can be (2) subscribed to for automatic retrieval of new episodes/content.