Read my thread in my signature. It will explain the kernels to you
As far as ROMs are concerned, they are like GUIs. They are the same thing, just look different. But then you have two categories of ROMs, AOSP and Sense ROMs. Generally AOSP roms are SUPER light on the memory and the size of the actually ROM. Sense ROMs tend to be memory hogs, and almost double the size ( of the ROM that is ).
I'll leave radios up to someone else. I could giv you the bsics, but its the more obvious answer haha