What Is A Podcast?
Here's the definition:
In a nutshell, a podcast is an audio file in MP3 format* and delivered online via RSS feed (webfeed) subscriptions. Podcast subscribers can listen to the show at his or her own leisure, which is the main attraction of this medium. As unlike Internet radio, there is no schedule to follow.
You need a podcast client/receiver to subscribe to a podcast. When you're subscribed, your client will check for new shows automatically when you ask it to. Then, you just listen to the podcasts on a computer or portable device (MP3 player, PDA or cellphone) when and where you want to.
Wikipedia also offers a comprehensive explantion of podcasts.