According to Strunk and White’s classic ELEMENTS OF STYLE, dashes are marks of separation “stronger than a comma, less formal than a semicolon, and more relaxed than parentheses.” Most often, they’re used to emphasize a pause, add or correct information, or signal an abrupt switch in thought or tone. The sentences below illustrate their handiness. Suggestions for alternate punctuation are also offered.

I need to purchase my return ticket for—oh, no, my wallet is missing! (change in thought)
After many sleepless nights—and some careful consideration—Grace decided to quit her job and start a business. (additional information; commas okay)
The contestant held his breath—had he given the game show host a correct answer? (pause for emphasis; semicolon also appropriate)
The ingredients for the cake—flour, sugar, vanilla, butter, and eggs—are on the kitchen counter. (adds list; parentheses acceptable)
The unhappy child opened her mouth and began to scream—a shrill, piercing, earsplitting wail. (emphasis; could use colon instead)

The “em dash”(its technical term) is formed by hitting the hyphen key twice. On most PCs, it can also be formed this way: type the first word, hold down the ALT key, type 0151 on the numeric pad, and then enter the next word. Like magic—there it is!

Keep in mind that using just dashes alone lessens their effectiveness. Mix in other punctuation marks when appropriate—commas, semicolons, colons, and parentheses—to vary the rhythm of written work.