umm... I believe you are reffering to two different things. Spinning your tires in one spot with out moving is only really bad for your tires. It can also mess with your diff, if you do it a lot, for instance a daily basis I would definately think about learning how to rebuild the diff and do it every month or so.
However, if you do it by standing on your brake and then flooring the gas pedal then releasing the brak, that is a very bad idea and hurts just about everything that is possible to hurt. For instance your brakes and whole driveline.