Xorg is the window system in most Unix-derivative OSs.
You can find more general info at x.org but honestly, it won't tell you anything about your bug.
Xorg has to be there. It's what lets the N900 display anything beyond basic text-based interfaces, as I understand it. (Obviously it's more complicated than that. But the point is it's more low level than Hildon Desktop, and killing that is typically bad - will cause instant reboot unless you take out the lifeguard flag in your N900 with the flasher utility.)
If Xorg is giving you CPU spikes/constant-over-activity, that means something else is malfunctioning and causing xorg to eat that much CPU.
See, my default recommendation whenever someone has any problem that could be caused by any number of things is: List everything you have installed. Literally, everything. If people did that all the time we'd be able to figure out the connections between odd rare bugs and certain programs a lot faster and more accurately.
I think your recommendation is the best way to find errors
