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errr, well unless I'm misunderstanding you:
Radio Stream - you listen to content that is streaming to your computer
PodCast - you fully download a file of content to your HD (and/or transfer it to your iPod) and then play it back.

Well that's the gist of it anyway..
Vanilla
 
Of course you can use a tool like RadioRecorder to record streaming internet radio - basically turning your streaming radio into a podcast. :D

It's a great way to grab shows that don't provide a downloadable format. I use it to listen to a number of NPR shows (e.g. "Car Talk", "Says You").
 
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