We use the dash ( — ) to:
1. set off parentherical information, just like we do with commas or parentheses. However, use dashes when you want to set off the information with greater emphasis than you could with commas or parentheses.
- You should also use them when the parenthetical information is too long to be set off by commas.
2. mark sharp turns in thought.
- I read most of this author's books: The Third Twin, Eye of the Needle, Pillard of the Earth — and loved each of them.
3. set off parenthetical information that already contains commas.
- We thought about moving to Los Angeles — an idea that, however realizable, had to be put off until next year — and starting a small business there.
4. introduce explanatory or appositive material.
- Many important personalities have come to the funeral — the President, the Prime Minister, and the Queen.
We can also use colons in similar context.
- Many important personalities have come to the funeral: the President, the Prime Minister, and the Queen.
5. emphasize a contrast.
- This particular model is rather cheap — but much less efficient.