Your typical Mac will start with something called Open Firmware, which loads the OS and things like that. Now, it is basically like a DOS shell: it can be used to type commands and boot into various OSes and run recovery programs. Now, this shell is accessible by any end-user who owns a Mac by pressing Command-Option-O-F when the Apple logo appears. You will see some text, and you will be dropped into a DOS-like shell. Here is a short list of commands you may use:
mac-boot -> boots into OS X
dev -> configures devices
ls -> Lists hardware configuration
dir -> does exactly as it does in DOS, lists files in a specified directory
devalias -> shows you the different aliases and what they're for
load -> loads a kernel into RAM
boot -> boots a loaded kernel
printenv -> shows the environment variables
setenv -> allows you to make and change system variables, namely Open Firmware ones
shut-down -> shuts down your Mac
reboot -> reboots your Mac into OS X, although you could sue mac-boot for that!
Thanks for reading this thread and if you know any more commands post them below please! :)
NOTE! I would not really recommend *messing* with Open Firmware as it can be very dangerous.