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 () (Control key)
(Option key) - "Alt" may also appear on this key
(Caps Lock) - Toggles Caps Lock on or off
fn (Function key)
Startup keyboard shortcuts
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 combinationWhat it does
OptionDisplay all bootable volumes (Startup Manager)
ShiftPerform Safe Boot (start up in Safe Mode)
CStart from bootable media (DVD, CD, USB thumb drive, and so forth) TStart in FireWire target disk mode
NStart from NetBoot server
XForce Mac OS X startup (if non-Mac OS X startup volumes are present) Command-VStart in Verbose Mode
Command-SStart in Single User Mode
See also: Startup key combinations for Intel-based Macs.
Finder keyboard shortcuts
Key combinationWhat it does
Command-ASelect all items in the front Finder window (or desktop if no window is open) Option-Command-ADeselect all items
Shift-Command-AOpen the Applications folder
Command-CCopy selected item/text to the...