Podcasting contains audio content or file which can be downloaded (By RSS feed or by streaming mp3). Formats can be mp3 or AAC (Apples iTunes format)
Podcasting though is lot like conventional broadcasting but it differs in audience delivery. The television, radio or internet radio broadcasting is done for general audience. But with pod casting the scenario is complete different
Here, the listener tunes into syndicated web site content, the content is available in audio format (mp3 or AAC) and you can listen to your favorite number on your iPod.
Wikipedia describes it as
Podcasting is a blanket term used to describe a collection of technologies for automatically distributing audio and video programs over the internet via a publish and subscribe model. Podcasting enables independent producers to create self-published, syndicated “radio shows,” and gives broadcast radio or television programs a new distribution method.
In the podcasting model, the publisher publishes a list of programs in a special format, known as a “feed”, on the web. A user who wants to see or hear the podcast subscribes to the feed in special “podcatching” software (a type of aggregator), which periodically checks the feed and automatically downloads new programs as they become available. Typically, the podcatching software also transfers the program to a desktop or portable media player.
