You can change the way that your mac boots by pressing & holding the following key combinations as the machine starts up
Option (ALT) – Display all bootable volumes
This will normally show you any internal hard drives, CD’s or DVD’s which are currently capable of booting from as well as external hard drives, USB keys, CD’s or DVD’s which could be used to boot the machine. If you have bootcamped your mac then this will enable you to choose between OS X or Windows.
If you have set a firmware password, then you will be presented with a padlock and a box to enter a password.
Shift – Safe Boot
This will disable any non system startup items, this can be very useful in troubleshooting a variety of issues.
C – Boot from CD
Hold this down until the machine boots from the CD, this can take a while sometimes so I would recommend the ALT method for this, this will be disabled if a firmware password is enabled.
T – Target Disk Mode
This will basically turn your mac into an external hard drive, connect it to another mac & you will have full access to all data on the machine, this will be disabled if you have a firmware password enabled.To boot into Target Disk Mode with a firmware password enabled, you will need to log in, and then select the Target Disk Mode option from within the Startup Disk preference pane located in System Preferences.
N – NetBoot
Startup from a NetBoot server.
X – Force Mac OS X Startup
This can be used if you have multi-boot machines and have set the default disk to be a non mac partition.
CMD-V – Verbose Mode
Verbose Mode shows you all that is going on whilst your machine is booting, very useful in troubleshooting as you can spot the faulting items.
CMD-S – Single User Mode
Command Line Interface, should only really be used for machines that are having serious issues, you can run a file system check (fsck) or Applejack if you have that installed.
CMD-R – Recovery Mode
On Machines with 10.7 installed then this will normally boot from the Recovery partition, although I believe the new Mac Mini Servers will connect to the internet and run a recovery over the net