The dash (--)
The dash is longer than a hyphen. There are in fact two different dashes: the en-dash is the same width as a letter N, while the em-dash is the same width as the letter M. Both of these can be found by on MS Word by going to: Insert, Symbol and then selecting the dash you require.
The dash can be used to set off parenthetical elements, when those elements themselves contain internal forms of punctuation. Use the em-dash in these situations.
Consider the following sentences:
My friends, Paul, Barry, Steve and Homer, all love rhubarb crumble.
My friends-Paul, Barry, Steve and Homer-all love rhubarb crumble.
The dash should not be used to set off parenthetical elements when a comma would do just as well. There needs to be a good reason to use the dash. The em-dash can also be used in direct speech to signal a break in thought or a shift in tone.
'What on earth can I do-,' Alan jumped up and ran to the door.
'I've just asked you to-oh what was I telling you?'
The en-dash (the em-dash's slightly slimmer cousin) is used for indicating the space between dates in a chronological range.
The Second World War (1939-1945) was one of mankind's darkest hours.