If you come from a Windows/Linux background, you will be used to boot loaders and boot keys for performing particular functions on bootup. Key's to access the BIOS, boot to Safe Mode and more.
While the Mac OS X boot process is streamlined and it hides the actual processes that are run, it does still include a number of special keys that you can use during the boot process to perform a range of useful functions.
These commands work in Leopard through to Mountain Lion. I'm sure they probably work in earlier versions and will likely work in later versions too.
Boot from a CD/DVD
Press and hold the C key as soon as the white boot screen appears or the startup sound plays to tell your Mac to boot from the optical drive.