By Richard Mallion
There are now quite a few different combination of keys you can hold down at startup that effects the way your Mac boots, so here is a useful summary.
Press C during startup: Start up from a bootable CD or DVD, such as the Mac OS X Install disc that came with the computer.
Press D during startup: Start up in Apple Hardware Test (AHT).
Press Option-Command-P-R until you hear two beeps: Reset NVRAM
Press Option during startup: Starts into Startup Manager, where you can select a Mac OS X volume to start from. Note: Press N to make the the first bootable Network volume appear as well.
Press Eject, F12, or hold the mouse or trackpad button: Ejects any removable media, such as an optical disc.
Press N during startup: Attempt to start up from a compatible network server (NetBoot).
Press T during startup: Start up in Target Disk Mode.
Press Shift during startup: Start up in Safe Boot mode and temporarily disable login items.
Press Command-V during startup: Start up in Verbose mode.
Press Command-S during startup: Start up in Single-User mode.
Press Option-N during startup: Start from a NetBoot server using the default boot image.
Press Command-R during startup: Start from Lion Recovery