Thanks lofreq, that's a great guide. So as I understand flashing the modem is just like changing the drivers for something on a pc? And doesn't change anything in the rest of the android os, and if I change rom it will completely be replaced? Right? I've still got a lot of reading to do before going on to any of this as you can tell :P
Not exactly - there's essentially 4 parts to what you see on the phone, and there are dependencies between them (to some extent):
Kernel: This is the heart of the Android O/S, and is the layer that sits between the hardware (CPU, screen, RAM etc) and the applications. It's responsible for managing resources so that apps always have enough to run correctly. This is why optimizing it the way Devs like Supercurio & Harcore do is so crucial to the speed of custom ROMs.
Base ROM (aka PDA): This is the Galaxy S-flavoured Android O/S effectively containing all the Galaxy S drivers, but unlike closed-system devs like Microsoft, it's compiled from Source Code that has been 'released' by Samsung. Because Doc gets the source code, he can do as he likes with it, so removes extraneous bits n pieces, optimizes it carefully, and adds other things like BLN notification etc before compiling it.
Modem (aka Phone): This is the piece of software that actually makes your phone a phone, so it controls voice functionality, as well as sms, mms, wi-fi, 3G, GPS etc.
CSC (Country Specific Code): This is a tiny bit of code that sets your APN settings to those of your local carrier, and also influences which apps you can see in the Market. There is some debate over whether it's actually necessary - not having an NZ CSC certainly doesn't seem to cause anyone any issues here in NZ.
Hope that helps - I was confused as hell when I started out, but you'll get the hang of it. You'll probably stuff a few things up along the way, but there's plenty of help to be found here!!
oh was looking for that! Much clearer than my hit and miss definitions hehe!