I’ve been asked a few times now what the difference is between a hyphen and a dash. So here’s the answer, perfectly answered at http://www.writingforward.com/ :

http://www.writingforward.com/grammar/punctuation-marks/punctuation-dashes-and-hyphens-whats-the-difference

Just to add that my own preference is to include a single space before and after the em dash to set off an element added to amplify or to digress from the main clause. (Click on the link above and read through in order to understand what I am talking about). Here’s an example:

“I couldn’t take part in the race — although I badly wanted to — because I had to do an exam on the same day”.

Note that my computer has no em dash option hence the use of two en dashes together.