A bootloader is a small piece of software that controls which applications are loaded in the handset. When you buy a device, the carrier and the handset maker have pre-loaded applications, many of which you cannot remove. With this, you will be able to device what is being loaded. That says, there’s also a remote possibility of “bricking” your handset.
It seemed that originally it was planned to be an OTA update, but many had speculated that there would be an issue with carriers who may not take too kindly with HTC allowing customers to unlock their device’s bootloaders. HTC later then revealed that users will be able to unlock their devices through the HTC Dev website, but did not mention as to when the unlocking tool would be arriving.
It looks like it’s finally here but the bad news is according to HTC Dev’s website, so far only the EU version of the HTC Sensation is able to be unlocked. There are more devices on the way naturally, such as the HTC EVO 3D and the Sensation 4G (T-Mobile version), and probably even more devices as time progresses.
If you live in EU or own an international version of the Sensation, just head on down to HTC Dev’s website to get your device’s bootloader unlocked. If you don’t see your device listed, there are other means to unlock its bootloader if you’re not fussy about it being an “unofficial” method. For more information, check HTC Bootloader FAQ