S-Off turns off the security flag of the boot loader. Allows you to flash more than what you can with S-On.
CID- carrier identification. For example, if you install a Google edition ruu, you would change the CID to receive OTA's
MID- model identification, not completely sure but I know certain files check this before installing, may need to change depending on what you want to install.
RUU: ROM update utility,either a .exe or .zip that reinstalls everything, if you have s-on, you can only install the same or newer version.
FUU- firmware update utility, basically the same as RUU
HBOOT- boot loader, or different version boot loader. Not 100% sure if you need to be S-Off
I'm not an expert, just know these simple things
Sent from my HTC One using XDA Premium 4 mobile app