The word Podcast comes from two words "iPod" and "Broadcast".
According to Wikipedia the definition of a podcast is:
"...a type of digital media consisting of an episodic series of audio files subscribed to and downloaded through web syndication or streamed online to a computer or mobile device. "
Nowadays a podcast is usually considered to be a series of audio or video files that you can subscribe to through free software like iTunes, so that when new episodes are released, your computer (or mobile device) will automatically synchronise and download the files for you to watch or listen to whenever it's convenient.
You can access all of the latest Zapp! English podcasts here.