A podcast is an episodic program delivered via the Internet using an XML protocol called RSS. Podcast episodes can be audio files, video files, documents, or any combination of the three. The publisher or broadcaster podcasts the program by posting the episodes and the XML document to a Web server. The consumer subscribes to the podcast using a client application, such as iTunes, which regularly reads the XML file and downloads new episodes.
Because podcasts can include several of the most common types of media, they can be used to offer many different kinds of programs. Any television or radio program can of course be transmitted as a podcast, as can any lecture, performance, or other recorded event. Using the (Apple iTunes) AAC file format, audio podcasts can include synchronized still images to create a narrated slideshow. In this way, a series of lectures can include the lecturer’s voice and presentation slides. In most podcasts, each episode is the same file format, so subscribers can enjoy the program the same way each time. However, some podcasts use multiple media. Language courses, for example, can combine video for a sense of the foreign country, audio for information and ear training, and documents for practice and reference.
From the iTunes FAQ: http://www.apple.com/itunes/podcasts/fanfaq.html