A RUU is a ROM update utility, and is a executable (run from a computer) provided by HTC. There is a RUU for each OTA update that exists. If you connect your phone to your computer via usb, and execute it, it will wipe your phone, and flash your phone to that OTA update, including the ROM sw version, a default data partition (all user apps wiped and flashed with default), the radio versions of that OTA (baseband/pri/nv/wimax), and hboot. If you have S-ON, you can only flash one if it is the same version that you have or newer. With S-OFF, you can flash whatever you want, but if you have hboot 2.02 or above, hboot will not be downgraded with everything else if the RUU is older. Basically, a RUU is a collection of img files that will overwrite all internal partitions.
A PC36IMG.zip file is a file that contains img files. These img files are designed to overwrite internal partitions just like a RUU. The difference is that a PC36IMG.zip file is flashed from hboot. Meaning that you place the PC36IMG.zip file on the root (top level) of your sdcard. It doesn't involve using a computer. When you turn the phone on and go to hboot, it will scan for files like this. If it finds one, it will prompt you to flash it. Another difference is that a PC36IMG.zip files can contain whatever img files you want. If you have S-ON, the same rules as a RUU apply. If you have S-OFF, you are free to flash whatever unsigned img files you want to whatever partitions you want. It's useful for flashing a different recovery, for example. In the case I was referring to, though, you can take all the contents of a RUU and put it in a PC36IMG.zip. Doing that will be exactly the same as flashing a RUU, but simpler (no computer involved). Again, the same rules apply. As you have hboot 2.02, if you flash a RUU or PC36IMG.zip file with the contents of a RUU that is older than hboot 2.02, you would end up with the older radio versions, but still have hboot 2.02.
Are you using a custom ROM right now or stock? If you go to menu->settings->about phone->software information, what do you see?