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.
If used indiscriminately as a substitute for semicolons, colons, commas, and periods, the dash will lose its intended effect.