Podcasting is a free service that allows Internet users to pull audio files (typically MP3s) from a podcasting Web site to listen to on their computers or personal digital audio players. The term comes from a combination of the words iPod (a personal digital audio player made by Apple) and broadcasting.
I see them here and there on HBO.
A podcast is an audio file, usually of a radio program, that can be accessed on the internet. It might be free, or by subscription.