Podcasting, a portmanteau of Apple's "iPod" and "broadcasting", is a method of publishing files to the Internet, allowing users to subscribe to a feed and receive new files automatically by subscription, usually at no cost. It first became popular in late 2004, used largely for audio files.
Think of it as a digital newsletter, in audio format, if you use iTunes it can automatically download the latest "episode" and download it to your iPod, or you can just listen to it.
en.wikipedia.org/wiki/Podcast