Scoble’s linkblog points to a good definition of podcasting from Wikipedia:
Podcasting is the creation and aggregation of discrete, downloadable media files. Typically, audio files are created on a computer and then uploaded to a web server on the Internet. An RSS 2.0 feed with enclosure tags is then amended each time a new media file is published, with a URL linking to each enclosure.
Client software (such as iPodder) is then used to aggregate the RSS feeds, automatically downloading the enclosures and organizing the files based on the user’s preferences.
The term podcasting is meant to rhyme with broadcasting and is a derivative of the iPod platform. While not directly associated with Apple’s iPod device or iTunes music service, they did both contribute the desire and the technology for this capability.
Given the nature of Wikipedia, this definition may change. The Wikipedia entry also inlcudes links to further information and resources about podcasting.