I never really understood the difference between using the hyphen, the en dash – and the em dash —. I found this article from list apart which explains it really well.

In a nutshell the basic uses are:

• Hyphen used to join two compound words (re-read)
• En dash used for a range of things such as dates, numbers or instead of to (2–9)
• Em dash used when writing a break in thought—like now!

Read the full explanation from alistapart here >
smashing magazine has a good explanation here too