Podcasts

I know what podcasts is and I've been running internet radio through shoutcast before (back when it wasn't as evolved as it is now), but I'm still new to this.

Does podcasting through blogs require the podcaster to upload the songs to the server? It sounds pretty dumb if that's the case. But how does the standard wordpress podcasting plugins work? Do they simply link to the users IP and mirror the playlist, or does it actually stream the music through the WP server?