A "podcast" is this: a posting of files, usually audio or video, set up so that it can be subscribed to through a Web feed (RSS) by a aggregator program. That's it. That's all it is. It can be a radio show, or an audio/video blog or pretty much anything.
It's a very basic tool given a fashionable, swanky, yet nearly nonsensical name (which comes from playing them on iPods) to make it seem special and magical. Like pretty much everything else on the internet. On the other hand, it's a simple, catchy name for a common phenomenon, much like 'television' and 'radio', both of which earned howls and jeers when they were new (the names and the phenomena).