A podcast is a simple audio or sometimes video file that provides you with any of a long list of items that can be used to teach you different languages, math, science, and even digital photography.
Some of the best guides and tutorials are podcasts.
Here are some of them that I listen to:
The main advantage of these podcast tutorials is that you can run them on your iPod or media player while you sit at your PC or in the field.
If you know of any others, please let me know and I can add this to the list.