ROM = Read Only Memory. We use ROM to name the file that is used to flash the device.
On the Zaurus, this ROM is a big file containing a snaphot of an entire filesystem (mean all files found in '/' ).
Kernel = the linux kernel without its modules. It's the kernel of the system, It's the first layer over the hardware. It's responsible of all exchange between all applications and between applications and device drivers.
The ROM (snaphot of an entire filesystem) contains all default applications but also modules (in /lib/modules/kernel-2.4.21/...). You can call them modules or loadable drivers. They could be part of the kernel but are not included by default to save space in memory and to improve the loading time.
Brinking the Zaurus = have a problem with the device (bad flashing, nand format to zero, no wake up...). Most people who said this have had simple error while flashing or their device didn't want to wake up because the battery was empty.
. For bad flashing : repeat the operation or see if your flash partition are well sized or use another kernel+ROM set.
. For device who doesn't want to wake up: connect the AC adaptator and keep it like this for at least 10 mn (my SL-C760 won't wake up without 10mn of battery charge if the battery is full discharged).
. For device bricked because of NAND format to zero (a dangerous Sharp system menu option) well... you will have to send your Zaurus back to repare. Most howto about this menu prevent you from using this dangerous option.
