Podcasting is an automatic mechanism by which multimedia computer files are transferred from a server to a client, which pulls down XML files containing the Internet addresses of the media files. In general, these files contain audio or video, but also could be images, text, PDF, or any file type.
A podcast is generally analogous to a recorded television or radio series.
The content provider begins by making a file (for example, an MP3 audio file) available on the Internet. This is usually done by posting the file on a publicly-available webserver; however, BitTorrent trackers also have been used, and it is not technically necessary that the file be publicly accessible. The only requirement is that the file be accessible through some known URI (a general-purpose Internet address). This file is often referred to as one episode of a podcast.
The content provider then acknowledges the existence of that file by referencing it in another file known as the feed. The feed is a machine-readable list of the URLs by which episodes of the show may be accessed. This list is usually published in RSS format (although Atom can also be used), which provides other information, such as publish dates, titles, and accompanying text descriptions of the series and each of its episodes. The feed may contain entries for all episodes in the series, but is typically limited to a short list of the most recent episodes, as is the case with many news feeds. Standard podcasts consist of a feed from one author. More recently multiple authors have been able to contribute episodes to a single podcast feed using concepts such as public podcasting and social podcasting.
The content provider posts the feed to a known location on a webserver. (Unlike the episode file itself, the feed is published to a webserver, usually not by other means.) The location at which the feed is posted is expected to be permanent. This location is known as the feed URI (or, perhaps more often, feed URL). The content provider makes this feed URI known to the intended audience.
A consumer enters this feed URI into a software program called a podcatcher, a type of aggregator. The former term is specific to podcasting while the latter is general to all programs which collect data from feeds. A podcatcher retrieves data from the feed URI and may process it further. (Early podcatchers are named in the History of podcasting page, but constantly updating a comprehensive list of software is beyond the scope of these encyclopedia entries.)
A podcatcher is usually an always-on program which starts when the computer is started and runs in the background. It manages a set of feed URIs added by the user and downloads each at a specified interval, such as every two hours. If the feed data has substantively changed from when it was previously checked (or if the feed was just added to the podcatcher’s list), the program determines the location of the most recent item and automatically downloads it to the user’s computer. Interestingly, it is estimated that perhaps only 20% of podcasts are actually consumed on portable media players; 80% are consumed on the PC onto which they are downloaded. Some podcatchers, such as iTunes, also automatically make the newly downloaded episodes available to a user’s portable media player. (This is only the typical behavior of a podcatcher; some podcatchers behave—or can be set to behave—differently.)
