A dash indicates a break in thought or the addition of information within a sentence or at its end. A dash is typed using two hyphens (although most word-processing programs can be set up to automatically insert a dash when you type two hyphens). There is no space before or after the punctuation mark.
- The woman came running around the corner—I couldn’t see her face—and disappeared down the alley.
- This building—and every building on the street—will be torn down. Marsha Nagib—you know her, I think—told me we might close early today.
A dash can be used to mean namely, that is, or in other words to introduce additional information or an explanation.
- I thought about taking another route—the one through West Virginia. There’s only one way to win—don’t play the game.