In the purest form of the term, Podcasts are published as RSS feeds (most blogging software features RSS feed generation). Listeners are notified of new programs by their aggregators, which then download the programs and then transfer them to the listener's MP3 audio player, such as an iPod.
Podcast interview with Rosemary Lehman and Simone Conceicao, authors of Creating a Sense of Presence in Online Teaching: How to "Be There" for Distance Learners.