To clarify previous posts:
: Resets the device to factory defaults or to the last rom you installed.
combo of hardware buttons (described above)
slide through HTC sence till "Settings" bar and select "Security" (top option). Then select "Factory resets" (last option). To my knowledge it does absolutely the same thing as the hardware method. correct me if I'm wrong.
: changes you made will take effect after the device resets (installed apps or reg edits...) OR you got to a sticky situation and you need to reset to get everything running smooth again (device shows abnormal operation of some kind).
Hold the power button till the quick list appears and press "power off" option on the top. You can also be presented with this option after installing some apps or after changing something within an app.
remove the back cover and press the little red button near the sd card slot. Useful if the device stops responding and you are unable to soft reset by any other way.
Enter Bootloader Mode
: allows for flashing devices in any battery state. It doesn't do anything by itself, just allows roms to be flashed. Some people have gotten stuck on this screen http://forum.xda-developers.com/showthread.php?t=599259
, so use the search tool and read up before using. After flashing a rom you will loose every setting and file you had on the device itself. SD card will remain untouched (or so it should). I've read people advising to remove the card prior to flashing. Not exactly sure as to why. Files saved to the SD will remain intact. Apps installed on the SD card (aka. "Storage") will remain intact, only not registered or linked with shortcuts anymore. The device basically doesn´t know they exist.
button combo (described above).
If you don't have to manually enter bootloader, the rom installer will take your device through this step without you having to do anything besides connecting through USB (activesynk, I guess) and running the RUU_LEO_xxx.exe file. You will see a progress bar on both PC and device as the rom is flashed.
Note: Reseting the device (turned on) while pressing a key (ex.: vol down) does the same thing as pressing the power button (with the device turned off) while pressing that same button (vol down).
Maybe you could transfer this info to the 1st post and clean up the thread a bit... Even, this could be at the Wiki section. Or better, a link on the Wiki to this thread