In ordinary day-to-day scribbling you may not need to go beyond the usual horizontal dash-hyphen at the right of the top row on your keyboard. For a more formal presentation, such as a letter to be printed out and sent by post or attached to an email, you may want to consider using 'proper' hyphens and dashes. If you write professionally, it may be the house-style requirement.
The three common types are as follows:
- Hyphen, it's on the QWERTY keyboard to the right of the zero.
– Longer, called n-dash. To type, press Option (Alt) key and type hyphen.
— Still longer, called m-dash. To type, press Option+Shift key and type hyphen.
Don't type two hyphens (--) to represent a long dash, it looks untidy and amateurish.
If you edit copy and want to replace short hyphens for longer dashes, open the search window (Command+F). In the search space at the top type Space+Hyphen+Space (not the words – just hit the space bar, then hyphen key, then space bar again!). In the Replace window below type Space+Option/Hyphen+Space. Then replace all or one by one.
Here is what the Oxford English Dictionary says:
dash – a horizontal stroke in writing or printing to mark a pause or break in sense, or to represent omitted letters or words.
hyphen – the sign (-) used to join words to indicate that they have a combined meaning or that they are linked in the grammar of a sentence (as in pick-me-up, rock-forming), to indicate the division of a word at the end of a line, or to indicate a missing or implied element (as in short- and long-term).
On the various and often confusing rules and styles on hyphens and dashes read an extensive article on wikipedia.
Read also 'Editing Large Texts: the Many Uses of Command+F Function'