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From WikiPedia:

"Podcasting is a means of distributing audio and video programs via the Internet that lets users subscribe to a number of files, also known as "feeds", and then hear or view the material at the time that they choose. A feed is usually in the MP3 audio format.

Podcasting became popular in late 2004 with the spread of free software that enabled automatic downloading of audio MP3 files onto computers and mobile MP3 players.

"Podcasting" is distinct from other types of online media delivery because of its subscription model, rather than one-time delivery. A series of files are delivered because subscribers want to get the information regularly."