Technically, you mainly use the em dash to set off explanatory or amplifying statements—if you really want them to stand off. But in practice, you can also use it to signify breaks in thought or to solve tricky grammatical problems. Not sure how to punctuate? Slap in an em dash.
Now, I don’t recommend using this very often, but the em dash can bail you out from time to time. Some editors hate them, but I prefer them to ellipses (. . .) every time.
The em dash:
- sets off explanatory or amplifying material: The store was closed—a sign hung on the door.
- shows breaks or sudden shifts in thought: I really need to get an oil change—and where did I put my keys?