Firmware is software built into your Mac itself that contains (among other things) enough logic to enable the computer to boot and to perform certain basic tasks even when no disk is present with an installation of Mac OS X. Intel-based Macs use a firmware framework called EFI (extensible firmware interface), whereas PowerPC-based Macs use an older standard called Open Firmware.
One of the features built into both types of firmware is the capability to limit the ways in which a Mac can be started. Ordinarily, you can hold down one of several keys during startup to boot from another volume (such as an external hard disk or an optical disc) or perform any of several other administrative tasks that bypass your Mac's normal startup process. Because these procedures can bypass any security features you've enabled on your main startup disk, Apple provides a way to disable all these special startup shortcuts. To do so, you activate a firmware password.
When the firmware password is set, holding down any of the following keys at startup has no effect:
C. Starts up from the internal optical drive.
D. On Intel-based Macs only, starts up from the Diagnostic volume of a Mac OS X Install DVD.
N. Starts up from a NetBoot server.
T. Starts up in Target Disk Mode (for Macs that have FireWire).
Shift. Starts up in Safe Boot mode.
+S. Starts up in Single-user mode.