1. Do not confuse a dash with a hyphen. A hyphen differs in both use and appearance from a dash. Dashes are used to give greater emphasis, to explain a statement, to replace a comma or comma pair, or to set off a parenthetical statement.
This 65-year-old African-American female did not return to the office for follow-up until today—in spite of my warnings to the contrary—and the squamous cell carcinoma on the nose has likely metastasized in the interim.
2. A hyphen (-) is shorter than a dash (—).