Hold the Option key during OS X’s boot to run the Startup Manager, a graphical tool from which you can select the volume you want to boot. If you’re a Boot Camp user, you can choose Windows from here, but you can also boot from external hard drives, thumb drives (perfect for installing Leopard from a disk image), or even network drives. In general you don’t need to do anything fancy to boot your Mac beyond pressing the power button, but there’s a lot you can do during with OS X’s advanced boot options (e.g., earlier today we discussed putting your Mac into target disk mode). You may not use these everyday, but knowing your startup options can come in very handy at the right time.