Explain that the SPL is basically just a boot loader.
Explain that the ROM is the Windows Mobile OS and included softare.
Explain that the Radio is used by SPL to talk to the ROM (or whatever it actually does), kinda like BIOS(?). Also be sure to say that if you put an incompatible Radio, your phone bricks cause it won't boot. You can't put on bad "BIOS" and then replace it. You can put on good one and change it later if you want though.
Also a link to a program that can grab your current phone's ROM and Radio and back them up to your computer would be useful.
Again, thank you so much for writing this! It's given me the confidence to try Flashing.