Use of Dashes

The em dash, often just called the dash, is the most commonly used of the dashes. To avoid confusion, no sentence should contain more than two dashes. A pair of em dashes sets off an amplifying or explanatory element. Commas, parentheses, or a colon may perform a similar function, but em dashes are often used for emphasis. Be careful not to overuse them though.
Example: the influence of three Impressionist artists—Monet, Sisley, and Degas—is obvious in her beautiful portraits.

An em dash may be used to separate a subject, or a series of subjects, from a pronoun that introduces the main clause. Example: Broken promises, petty rivalries and false rumors—such were the obstacles they kept her from advancing in the company.

An em dash, or a pair of em dashes, can indicate a sudden break in thought or an interruption in dialogue.
Examples: Will he—can he—win the race?
“I don’t know what to say,” I began softly, “I thought I might—“