A dash is written as two hyphens typed together, with no space before or after either word.
1. Use a dash to set off information which you want to emphasize at the end of a sentence.
This is one way to handle an afterthought.
Example: More than anything, Joan wanted a family — and her biological clock was ticking.
Note: Some Word programs are now designed to automatically change a doubled hyphen into a dash. In the formatting lists, this mark is called an “M dash”.
2. Use a pair of dashes to insert information which you want to emphasize into a sentence.
Even a whole sentence can be inserted into another sentence in this manner.
Example: The same is true of writing a paper — you have to do it because of the teacher — but where and how do you begin?
3. Use a dash before a list much as you use a colon, but for stronger emphasis.
See Colon Rule #2 above.