Or an en dash . . . or a hyphen.
And what are those three things anyway?
Em dashes can indicate a sudden break in thought, most often in dialogue or narrative. They are also used in dialogue to show that one person has been interrupted by another. Em dashes can set off an explanatory comment, when a comma isn't enough but using parentheses doesn't seem to work either. (Yes, these are definitely judgment calls.) Here are examples of each:
Understanding the differences between em dashes, en dashes, and hyphens makes it easier to know when to use them. Each has a valuable job to do in our writing.