What is a podcast? Wikipedia.org says:

A podcast (or non-streamed webcast) is a series of digital media files (either audio or video) that are released episodically and often downloaded through web syndication. The word usurped webcast in common vernacular, due to rising popularity of the iPod and the innovation of web feeds.

Confused? Try this simple video to explain:

Podcasts are just a popular way to get audio content out to an audience on a regular basis. Once created audio content can be subscribed to and delivered automatically to your listeners. iTunes, a very popular music player from Apple, can easily subscribe to many podcasts and can be synchronised with Apple Ipod hardware. But there are may ways to consume podcasts that do not require iTunes or iPods. Many other devices can download and play podcasts especially the ever growing market of smart phones and tablet computers although one of the most popular ways of listening to the audio is directly through a web browser.

What can you podcast about?
Podcast subjects are many and varied but can include:

Business Podcasts
Training Podcasts
Coaching and Mentoring Podcasts
Music Podcasts
Hobbies and interests Podcasts