A dash (–) looks like a longer hyphen (-). A pair of dashes is used to enclose a strong interruption/interjection, separating it from the rest of a sentence. If the interruption comes at the end of a sentence, then only the first dash is needed (the second is assumed). Usually, we use a dash for shock or irony.
Examples:
Dashes can also be used in ranges or sequences, such as 75–100 kg, 13–17 May, or an A–Z.
A tip: if you've more than four pairs of dashes on a page, check to see that they are really the best form of punctuation. Often, commas are more appropriate.