Mac OS X Tiger/System Preferences/Startup Disk

This is where you decide how your computer should boot. The amount of options depend on your setup and most users will have only one choice, the OS X system on the internal harddrive. Other potential options are to boot from an official OS X disc (either one of the discs you got with the computer or the CDs you get if you buy OS X) or to boot from a BootCamp partition. It's also possible to have an extra OS X on an external harddrive and boot from that one. It's even possible to boot from a volume on a network.

This preference pane is very easy to use. Click the volume you want to have as start disk and press the Restart... button. The computer will restart and boot from the system you picked.

This is also where you restart in Target Disk Mode to be able to use the computer as an external harddrive (only works with FireWire).