A Radio stores the low level firmware for the desire's hardware (display, wifi, bluetooth, and everything else). Newer Radios contain newer firmware.
A RIL is like a driver pack that is optimized to work with a specific radio. So if you are running a certain Radio, say Radio XYZ, then for the best performance (ie wifi signal strength, battery life, raw performance), you would want the corresponding RIL (which would happen to be the RIL for XYZ.
Running a specific Radio's RIL is not necessary, but recommended.
A word on bumping - There are lots of knowledgable people here on XDA; perhaps it is unwise to 'diss' them simply because you haven't received a reply in under a day. Relax, someone will always answer your question, except (in the case of this thread) when the question being asked has been answered before, multiple times (i know this as i have answered it myself a couple of times).
