Dashes can be used in three ways – to introduce parenthetical information (emphasizing informal afterthoughts), to set off a series or list of items, or to indicate shifts or breaks in thought, speech, or action.
To introduce parenthetical information
The cost of the trip – the airfare in particular – is substantial.
To set off a list of items or a series
The unpleasant contents of the refrigerator – sour milk, moldy bread, and brown lettuce – made John shudder in disgust.
Squirrels, possums, raccoons – these animals are often victims of senseless speeding.
To indicate shifts or breaks in thoughts, speech, or action
She was a great mother – when she was out of town.
Susan left her car door unlocked – normal in Marquette – and quickly made her way inside the store.
"I wonder if – never mind," she said.
Commas usually are used to set off inserted material, but in instances where the insertion itself contains commas, dashes are preferable.
Perrin, Robert. The Beacon Handbook and Desk Reference. Boston: Houghton Mifflin Co., 2003.
Rosa, Alfred. The Writer’s Pocket Handbook. New York: Pearson Education, Inc., 2003.