!!WARNING!! THIS CAN GO WRONG AND MAY MAKE YOUR PHONE AS MUCH WORTH AS A BRICK! DO IT ON YOUR OWN RISK!
A quick Video tutorial on how to root your HTC desire using unrevoked.
This is propably the easiest way root your phone. It's quick, uncomplicated and the programm does pretty much everything on it's own.
Rooting your device will cost your warranty!
If you root your Htc desire you have much more options in your phone available. You can install custome Roms that drain less power, run smoother or have additional features für your phone. Installing an HTC desire HD rom on an HTC desire for example will give you the option of making 720p pictures and videos with your desire.