Posting this here for my own use:

⌘/Apple/Command:
This is known as the “Command” key, as in “press Command-q to quit.” This can be used to access most major menu items by pressing the key combination listed next to those items in the menu.
There are several standard commands which are consistent across nearly all Mac applications, such as ⌘Q (Quit), ⌘W (Close), ⌘C (Copy), ⌘V (Paste), ⌘X (Cut), ⌘O (Open), ⌘A (Select All), ⌘N (New), and ⌘-? for Help. ⌘-Tab quickly switches between applications. Learning these shortcuts can save you a lot of time.
Command-clicking can also be used to select multiple items at once.

⇧/Shift:
There aren’t enough letters to match every menu item, so shift is generally used in combination with ⌘ for less-commonly accessed menu items.
Shift-clicking can also be used to select a range of items.

⌥/Option/Alt:
This is known as the “Option” key. This is used in combination with ⌘ and ⇧ for even lower-priority menu items, or for variations on standard items. (For instance ⌘⌥W will close all of an application’s windows, instead of just one.) If you pull down a menu and then press option, the menu items will change to their variant options in real time.
In the menus, option is represented by “⌥”, presumably because there weren’t any other symbols left. (The key has “alt” printed on it for compatibility with Windows applications.)
When ⌥ is used as a modifier without ⌘, it accesses alternate characters. For instance, ⌥-2 gives you ™, and pressing ⌥ in conjunction with vowels lets you place accents over international characters.
Option-dragging items will copy them instead of moving them.
You can also use option with the arrow & delete keys to work with entire words at a time.

^/Ctrl/Control:
The control key exists primarily for compatibility with Windows and Unix applications. You won’t generally use it in Mac applications. If an application does include a control-key shortcut, it will be abbreviated as “^”.
Control-clicking is equivalent to a right-click.

Fn/Function:
The function key is unique to notebook computers, and exists solely to access secondary functions on the keyboard. Use this to access Home, End, Page Up, Page Down, the F1-F7 keys, and the number keypad.

source: ask.metafilter.com: