A podcast is a media object consisting of audio and/or video files shared over the internet. The podcast can be downloaded and consumed on a variety of audio/visual devices, including computers and portable devices such as ipods. The most distinguishing feature of a podcast is that consumers can subscribe to a podcast "feed" by entering the web address into an "aggregator," such as itunes. The aggregator automatically downloads new podcasts when they become available.
Unlike conventional radio shows, there is no "live" feed or broadcast involved in podcasting. Because podcasts are prerecorded, producers and consumers both create and consume at their convenience.Podcasts, as well as other digital media, promote the active use of file sharing, effectively producing a community based around the sharing of digital audio. Podcasts are important because they can express ideas that visual media cannot, simply beause a person must rely solely on their ears to absorb the material. Podcasts exemplify how digital audio interacts with listeners. For example, digital audio can affect the space that a listener listens in by compressing distance between people who must listen, thus providing a public experience. Or, with Ipods, listening experiences offer mobile privitization.
"A podcast is distinguished from other digital media formats by its ability to be syndicated, subscribed to, and downloaded automatically, using an aggregator or feed reader capable of reading feed formats such as RSS or Atom." 
- http://www.podcastdirectory.com/ A Directory of Popular Podcasts