A podcast is an audio or video digital-media file which is distributed over the Internet and can be listened to in portable media players and personal computers. Further information can be read in these Wikipedia entries: in English / in Spanish.
Podacastsinenglish.com is an example of website focused on English learning through podcasts.
This other site collects a series of podcasts recorded by children. The result is an audiobook with a version of Homer's Odyssey in Spanish.
Simpler use of podcasts can be found in blogs. This example of a post in a classroom blog presents students' recordings in Basque about some animal categories.
This video can help us understand the characteristics and uses of podcasts: