“A podcast is a media file that is distributed over the Internet using syndication feeds, for playback on portable media players and personal computers.” Wikipedia
“A podcast is a digital audio file (usually MP3 or AAC) made available for download on the internet through an RSS 2.0 feed.” How to Podcast - Technical definition
The term podcast comes from the use that early podcasters made of Apple MacIntosh's popular MP3 player the iPod. But...
You subscribe to them using an application such as iTunes,
Podomatic or Odeo.
Or they can be used for putting oral postings on your blog.