A podcast (or non-streamed webcast) is a series of digital media files (either audio or video) that are released episodically and often downloaded through web syndication.
The word usurped webcast in common vernacular, due to rising popularity of the iPod and the innovation of web feeds.
The mode of delivery differentiates podcasting from other means of accessing media files over the Internet, such as direct download, or streamed webcasting.
A list of all the audio or video files currently associated with a given series is maintained centrally on the distributor's server as a web feed, and the listener or viewer employs special client application software known as a podcatcher that can access this web feed, check it for updates, and download any new files in the series.
This process can be automated so that new files are downloaded automatically. Files are stored locally on the user's computer or other device ready for offline use, giving simple and convenient access to episodic content.
Commonly used audio file formats are Ogg Vorbis and MP3. In many respects, this is closer to traditional publishing models associated with books and magazines (as opposed to radio, which uses a live stream)
Check out http://podcast.com/