It can be just audio too. It just depends on the podcast. It's like radio (or tv if it's video), but with no commercials or censorship (as of yet), so you can imagine all the different types of podcasts one could listen to. Some do have commercials to help facilitate the production costs and give recognition to parties who've contributed.
Personally, I don't like the video ones if it's for instructional purposes because the ipod screen is so small it's worthless trying to see what they're pointing at. Of course, you don't have to watch it on the ipod - you can still view it on your PC but that defeats the portability purpose.