Source : support.apple.com
To use a keyboard shortcut, or key combination, you press a modifier key with a character key. For example, pressing the Command key (the key that has a symbol) and the "c" key at the same time copies whatever is currently selected (text, graphics, and so forth) into the Clipboard. This is also known as the Command-C key combination (or keyboard shortcut).
A modifier key is a part of many key combinations. A modifier key alters the way other keystrokes or mouse clicks are interpreted by Mac OS X. Modifier keys include: Command, Control, Option, Shift, Caps Lock, and the fn key (if your keyboard has a fn key).
Here are the modifier key symbols you can see in Mac OS X menus:
(Command key) - On some Apple keyboards, this key also has an Apple logo ()
(Option key) - "Alt" may also appear on this key
(Caps Lock) - Toggles Caps Lock on or off
fn (Function key)
Press the key or key combination until the expected function occurs/appears (for example, hold Option during startup until Startup Manager appears, or Shift until "Safe Boot" appears). Tip: If a startup function doesn't work and you use a third-party keyboard, connect an Apple keyboard and try again.
|Key or key combination||What it does|
|Option||Display all bootable volumes (Startup Manager)|
|Shift||Perform a Safe Boot (start up in Safe Mode)|
|C||Start from bootable media (DVD, CD, USB thumb drive, and so forth)|
|T||Start in FireWire target disk mode|
|N||Start from NetBoot server|
|X||Force Mac OS X startup (if non-Mac OS X startup volumes are present)|
|Command-V||Start in Verbose Mode|
|Command-S||Start in Single User Mode|