Em dash
An em dash marks a change in though, an interruption, an aside, or an appositive. It is called an em dash because it it the same width as an “M.” There are no spaces before or after an em dash. A question mark or exclamation point (but not a period) can appear before an em dash where appropriate.

Usage
1. An em dash precedes a quote attribution.

Example: “Description begins in the writer’s imagination, but should finish in the reader’s.”
—Stephen King, On Writing: A Memoir of the Craft

2. An interrupted thought or unfinished sentence

Example: Cathy said, “Please stop. Don’t—”

3. An em dash can set off parenthetical material

Example: His dog ran—tail wagging furiously—to greet him at the door.

4. A break in thought continuity

Example: Two stone gargoyles—Cassie had never seen something so hideous—guarded the entrance.

5. An abrupt, startling, or emphasized appositive

Example: Marcus—blood dripping down his face—walked into the kitchen.

6. To add emphasis on the last item in a list (often a jump in continuity from the previous elements)

Example: Mary hasn’t seen Jordan since the day he walked out the door and left her with crushed dreams, unpaid bills, and no car—and a baby on the way.

7. An aside

Example: I cleaned my fourteen-year-old son’s room—what a nightmare!—only to have it look like a tornado swept through it two days later.

8. A divided quote (interrupted by something narrative rather than attributive)

Example: “I’m leaving”—she took the car keys off the counter—”whether you like it or not.” (She is performing the action while speaking, but her words are spoken continuously.)

8A. If the actual quote is interrupted, the em dash goes inside the quotes

Example: “Because—” Mary shrugged her shoulders “—I don’t know what else to do.” (Mary’s spoken words are paused during the action.)