Dashes have three uses:
1. To set off parenthetical material that you want to emphasize; in other words, if you want to include information that is not absolutely necessary in forming a complete sentence but that is important to the idea you are communicating, introduce this information into the main sentence between dashes. Eg:
Her taste in music–from country to rap–exemplifies her eclectic personality.
2. Use dashes to set off appositives that contain commas; in other words, if you are renaming a nearby noun with something that contains commas, use dashes to set if off. (For more on using commas for appositives, see "Commas.") Eg:
Joe–a student who is also an athlete, actor, and writing coach–does not have enough time to join the committee.
3. Use a dash to prepare for a list, a restatement, an amplification, or a dramatic shift in tone or thought. Eg:
The vegetarian gasped in horror when he saw lining the wall of the cabin a collection of animal heads–moose, deer, bears, squirrels, all dead.
Notes: Do not overuse dashes or use them in place of other punctuation. Do not leave space before or after a dash.
1. Use parentheses to enclose supplemental material (including in-text citations), minor digressions, and afterthoughts. Eg:
We read Austen’s Northanger Abbey (her first novel) in our Eighteenth-Century Literature class.
2. Use parentheses to enclose letters or numbers labeling items in a series.
When driving to and from school, she always made sure to have the following in her glove compartment: (a) registration, (b) first-aid kit, (c) AAA card, (d) a phone card, and (e) gas money.