Podfic question..

[ash48]

There may be a more appropriate place to ask this question, like a podfic comm, but I'm not sure there is one. So I'll try here first.

I listen to my podfics on my MP3 player. So I DL the MP3 file when choosing a story to listen to. I've recently noticed that most stories are in M4b format. What is that? I can't seem to convert it to MP3 (I've tried through Sound Converter and it doesn't recognise the format). Is that just for iPods or something? Also - and here's a really naive question - What's a podbook? How does it differ from a normal podfic dl?

Thanks if any one can shed some light on my lack of technical knowledge.