A lot of Podcasts are taken from real radio shows broadcast "on the air". The whole show is recorded and then saved as a downloadable digital file (as an MP3). Sometimes the show is edited down (for instance one BBC Radio show is edited to remove ALL the music and just leave the inane drivel the presenter and his guests spoke during the show). Others are full unedited versions of what is broadcast.
What makes it a podcast rather than an online "Listen again" is two factors. One, it is designed to be in a downloadable format rather than "streaming audio" and two, it is stored in a web format which allws certain types of software to automatically download the latest podcast automatically. Essentially the computer links to a web page which tells the software the name and location of the podcast files and the software recognises that a new file has been added for it to download.
The information which tells the software where to look is known as an RSS Feed and is basically a list of information. It can be loaded into a web browser and appears as a page of text showing a "script" which (amongst other things) lists the files, their name and their location. If you have that information, you can download the files manually without using one of the Podcast readers.

Podcasts can contain audio or video material. The content can be almost anything from a standard radio show to an obsessed geek's ramblings. There are things such as tutorials for playing an instrument, college campus radio, commercial national broadcasters and anything in between. The only problem is locating what you want to hear. If you like litsening to radio, you will probably get along okay with podcasts. Of course, one just downloaded onto your computer ties you to where you can listen, but copy it onto an MP3 Player and you can have it plugged into your ear wherever you are.

There have been a number of threads where people have recommended specific podcasts. Try a forum search and then look at what has been mentioned. Podcasts are only as good or as bad as the people who put them together.
Quack!
GtD.