A podcast is a way to broadcast multimedia files—audio or video—over the Internet for playback on mobile devices (ex. MP3 players) and personal computers. Podcasting lets the podcaster create a broadcast about a subject of interest to him/her and their subscribers. Subscribers simply download the podcast and listen to it at their convenience.
Apple pretty much introduced podcasting when they developed the iPod and and iTunes
File types supported: mp3 or m4a for audio, mov or m4v for video
More information at: http://www.apple.com/itunes/store/podcastsfaq.html