Kernel vs ROM ?

[brazen]

Help the n00b please!

I'm new here from the Touch Pro threads. On WinMo devices we only had to flash ROM's, maybe a radio. I don't understand this talk about kernels. Can anybody explain the difference between a ROM and a kernel? Are they interchangeable? For example, I've flashed Modaco's recent cdma ROM....but did that install his kernel as well? How do I know that kernel I'm running? Can you simply re-flash a new kernel over an old one even if its not stock or is there anything else that needs to be done first? Can I use the modaco ROM w/ the Gumbo kernel? etc.

Also, are updated radio rom's available?

Lastly, which rom/kernel do you recommend and why? As far as I can tell we only have 3 cupcake cdma rom's to choose from at the moment: Modaco, Fresh & Gumbo....right? Also, which kernel is best?