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).
Academics at the Community, Journalism & Communication Research group at the University of Texas at Austin in the USA are proposing a four-part definition of a podcast: A podcast is a digital audio or video file that is episodic; downloadable; programme-driven, mainly with a host and/or theme; and convenient, usually via an automated feed with computer software.
The word “Podcast” is a blend of the words “pod”— from the name of Apple’s portable media player, iPod—and “broadcasting”.The name may be misleading, as despite the etymology it has never been necessary to use an iPod, or, indeed, any other form of portable media player, to use podcasts; the content can be accessed using any computer that can play media files. Use of the term “podcast” predates the addition of native support for podcasting to the iPod, or to Apple’s iTunes software. To avoid a term suggestive of “iPod”, some use the term netcast instead of podcast, such as the TWiT.tv podcaster Leo Laporte. A backronym has been posited where podcast stands for “personal on demand broadcast
Source: http://en.wikipedia.org/wiki/Podcast