These are the terms of the HTC android ruu
RUU*– Rom Update Utility. It is a Windows based exe file that contains a complete operating system including a stock recovery, bootloader (Hboot), and radio (Baseband.) This program is run to bring the phone back to a stock state while the phone is in bootloader mode.
OTA*– Over the air update. It is an update package used to update a stock operating system. It is not a complete operating system and contains only parts that are getting an update . You can’t install an OTA on a modified device, only stock.
Bootloader (Hboot)*– The Bootloader or Hboot, is like BIOS to windows. It is the first thing that runs when you boot up your Android device. It packages the instructions to boot the operating system kernel and most are specifically designed to run their own debugging or modification environment. Basically, an Android device is like a hard drive that has several partitions, which holds the Android system file in one and all the app data in another. For all these partitions, Bootloader (or Hboot) serves as a security checkpoint that is responsible for checking and initializing the hardware and starting software. There's a common misconception that can you just change the hboot but that isn't the case, you need the full firmware that comes with the RUU.
Bootloader mode / Fastboot mode*– One and the same. This is the environment where you can flash a custom recovery (the stock recovery has very little options), flash Roms, etc. To enter Bootloader mode, power down the phone and then hold the power key and the volume down key at the same time until the phone enters bootloader mode. If your computer is not connected you will see “Fastboot” highlighted in red. If your computer is connected you will see “Fastboot USB” highlighted in red. This still does not mean that your computer can communicate with the phone. More on that later.
Firmware*– The image version i.e. 3.18.502.6, 2.20.502.7 etc. The firmware contains the Hboot so don’t confuse the Firmware version number with the Hboot version number or Baseband version number. You can check this out in more detail while looking at the RUUs available and it will make more sense. Each software version is connected with the CID (Carrier ID) and MID (Model ID) of your device. That means you can't use whichever version you want, unless you have S-OFF on your device. Each ROM Update Utility.exe package will check your device details (bootloader version, MID and CID) before starting the update process.
Baseband (Radio)*- The baseband (radio) is completely separate from the ROM, it's stored in an entirely different partition. Don’t confuse this with Hboot or Firmware