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/trackpad clicks are interpreted by OS X. Modifier keys include: Command, Shift, Option, Control, Caps Lock, and the Fn key. If your keyboard has an Fn key, you may need to use it in some of the key combinations listed below. For example, if the keyboard shortcut is Control-F2, press Fn-Control-F2.
Here are the modifier key symbols you may see in OS X menus:
| ⌘ | Command key |
| ⌃ | Control key |
| ⌥ | Option key |
| ⇧ | Shift Key |
| ⇪ | Caps Lock |
| Fn | Function Key |
Startup 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, try again with an Apple keyboard.
| Key or key combination | What it does |
| Option | Display all bootable volumes (Startup Manager) |
| Shift | Perform a Safe Boot (start up in Safe Mode) |
| Left Shift | Prevent automatic login |
| 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) |
| D | Use Apple Hardware Test |
| Command-R | Use Recovery (OS X Lion or later) |
| Command-V | Start in Verbose Mode |
| Command-S | Start in Single User Mode |
| Command-Option-P-R | Reset NVRAM / parameter RAM |
| Hold down the Media Eject (⏏) key or F12 key, or mouse or trackpad button | Eject removable discs |
Sleep and shut down shortcuts
| Key or key combination | What it does |
| Power button | Turn off your Mac or put it to sleep |
| Hold down the power button | Force your Mac to shut down |
| Command-Control-power button | Force your Mac to restart |
| Control-Media Eject (⏏) | Show restart/sleep/shutdown dialog |
| Command-Option-Media Eject (⏏) | Put the computer to sleep |
| Command-Control-Media Eject (⏏) | Quit all applications (after giving you a chance to save changes to open documents), then restart the computer |
| Command-Option-Control-Media Eject (⏏) | Quit all applications (after giving you a chance to save changes to open documents), then shut down the computer |
| Shift-Control-Media Eject (⏏) | Put all displays to sleep |
Finder keyboard shortcuts
| Key combination | What it does |
| Command-A | Select all items in the front Finder window (or desktop if no window is open) |
| Command-Option-A | Deselect all items |
| Command-Shift-A | Open the Applications folder |
| Command-C | Copy selected item/text to the Clipboard |
| Command-Shift-C | Open the Computer window |
| Command-D | Duplicate selected item |
| Command-Shift-D | Open desktop folder |
| Command-E | Eject |
| Command-F | Find any matching Spotlight attribute |
| Command-Shift-F | Find Spotlight file name matches |
| Command-Option-F | Navigate to the search field in an already-open Spotlight window |
| Command-Shift-G | Go to Folder |
| Command-Shift-H | Open the Home folder of the currently logged-in user account |
| Command-I | Get Info |
| Command-Option-I | Show Inspector |
| Command-Control-I | Get Summary Info |
| Command-Shift-I | Open iDisk |
| Command-J | Show View Options |
| Command-K | Connect to Server |
| Command-Shift-K | Open Network window |
| Command-L | Make alias of the selected item |
| Command-M | Minimize window |
| Command-Option-M | Minimize all windows |
| Command-N | New Finder window |
| Command-Shift-N | New folder |
| Command-Option-N | New Smart Folder |
| Command-O | Open selected item |
| Command-Shift-Q | Log Out |
| Command-Shift-Option-Q | Log Out immediately |
| Command-R | Show original (of alias) |
| Command-T | Add to Sidebar |
| Command-Shift-T | Add to Favorites |
| Command-Option-T | Hide Toolbar / Show Toolbar in Finder windows |
| Command-Shift-U | Open Utilities folder |
| Command-V | Paste |
| Command-W | Close window |
| Command-Option-W | Close all windows |
| Command-X | Cut |
| Command-Option-Y | Slideshow (Mac OS X v10.5 or later) |
| Command-Z | Undo / Redo |
| Command-1 | View as Icon |
| Command-2 | View as List |
| Command-3 | View as Columns |
| Command-4 | View as Cover Flow (Mac OS X v10.5 or later) |
| Command-Comma (,) | Open Finder preferences |
| Command-Accent (`) (the Accent key above Tab key on a US English keyboard layout) | Cycle through open Finder windows |
| Command-Shift-Question Mark (?) | Open Mac Help |
| Command-Left Bracket ([) | Go to the previous folder |
| Command-Right Bracket (]) | Go to the next folder |
| Command-Up Arrow | Open the folder that contains the current folder |
| Command-Control-Up Arrow | Open the folder that contains the current folder in a new window |
| Command-Down Arrow | Open highlighted item |
| Command-Shift-Up Arrow | Make the desktop active |
| Right Arrow (in List view) | Open the selected folder |
| Left Arrow (in List view) | Close the selected folder |
| Option-click the disclosure triangle (in List view) | Open all folders within the selected folder |
| Option–double-click | Open a folder in a separate window, closing the current window |
| Command–double-click | Open a folder in a separate window |
| Command-click the window title | See the folders that contain the current window |
| Command-Tab | Switch application–cycle forward |
| Command-Shift-Tab | Switch application–cycle backward |
| Command-Delete | Move to Trash |
| Command-Shift-Delete | Empty Trash |
| Command-Shift-Option-Delete | Empty Trash without confirmation dialog |
| Space bar (or Command-Y) | Quick Look (Mac OS X v10.5 or later) |
| Command key while dragging | Move dragged item to other volume/location (pointer icon changes while key is held- |
| Option key while dragging | Copy dragged item (pointer icon changes while key is held- |
| Command-Option key combination while dragging | Make alias of dragged item (pointer icon changes while key is held–see |
Universal Access VoiceOver keyboard commands
| Key combination | What it does |
| Command-F5 or Fn-Command-F5 | Turn VoiceOver on or off |
| Option-Control-F8 or Fn-Option-Control-F8 | Open VoiceOver Utility (if VoiceOver is on) |
| Option-Control-F7 or Fn-Option-Control-F7 | Display VoiceOver menu (if VoiceOver is on) |
| Option-Control-Semicolon (;) | Enable/disable VoiceOver Control Option-lock |
| Command-Option-8 | Turn Zoom on or off |
| Command-Option-Plus (+) | Zoom In |
| Command-Option-Minus (–) | Zoom Out |
| Command-Option-Control-8 | Invert/revert the screen colors (OS X Lion or earlier) |
| Command-Option-Control-Comma (,) | Reduce contrast (OS X Lion or earlier) |
| Command-Option-Control-Period (.) | Increase contrast (OS X Lion or earlier) |
Full keyboard access
Full keyboard access lets you use your keyboard to navigate and interact with items on the screen. Use these shortcuts to select and adjust controls such as text fields and sliders.
| Key combination | What it does |
| Tab | Move to the next control |
| Shift-Tab | Move to the previous control |
| Control-Tab | Move to the next control when a text field is selected |
| Arrow keys | Move to the adjacent item in a list, tab group, or menu or Move sliders and adjusters (vertical up and down arrows used to increase and decrease values) |
| Control-Arrow keys | Move to a control adjacent to the text field |
| Space bar | Select the highlighted menu item |
| Return or Enter | Click the default button or perform the default action |
| Esc | Click the Cancel button or Close a menu without choosing an item |
| Key combination | What it does |
| Left Arrow and Right Arrow | Move from menu to menu |
| Return | Open a selected menu |
| Up Arrow and Down Arrow | Move to menu items in the selected menu |
| Type the menu item’s name | Jump to a menu item in the selected menu |
| Return | Select a menu item |
Universal Access – Mouse Keys
| Key combination | What it does |
| 8 | Move Up |
| 2 | Move Down |
| 4 | Move Left |
| 6 | Move Right |
| 1 | Move Diagonally Bottom Left |
| 3 | Move Diagonally Bottom Right |
| 7 | Move Diagonally Top Left |
| 9 | Move Diagonally Top Right |
| 5 | Press Mouse Button |
| 0 | Hold Mouse Button |
| . (Period on number pad) | Release Hold Mouse Button |
Source: http://support.apple.com