Source : support.apple.comTo 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 ()
 (Control key)
 (Option key) – “Alt” may also appear on this key
 (Shift 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.

See also: Startup key combinations for Intel-based Macs.