"Any software and hardware combination that permits automatic downloading of audio files (most commonly in MP3 format) for listening at the user's convenience."
Podcasts use the RSS feeds technology that searches for exact content you specified in your search and deliver the content at your desktop.
In simple terms, "a podcast is the name given to a file that is a voice recording for playback on a computer, or saved onto an MP3 digital audio player and listened to at some future point".
Bradley, Phil . How to use web 2.0 in your library. London: Facet Publishing, 2007.
Educause http://www.educause.edu/ir/library/pdf/ELI7003.pdf
