Dashes and parentheses set apart phrases or clauses more aggressively than pairs of nonrestrictive commas.
Dashes
Use dashes, whether singly or in tandem, to indicate any sudden break in thought or construction.
The book that he slid from the shelf — that very book in your hand — holds the secrets of a good life.
I would like to purchase that book — how much did you say it cost?
The commotion within the classroom — commotion is a polite way to explain it — caused the professor to reach for the headache powder.
Use dashes in place of commas to set off some part of a sentence more forcefully.
Take a look — take a really good look — at the difficulties before you decide to follow your heart.
What constantly eludes me, I have always needed most — peace and quiet.
Don’t use dashes where they are not required or in place of some other more appropriate mark of punctuation. Dashes produce a special effect — make sure that you need that effect.
Parentheses
Use parentheses to enclose a statement that is useful but introduced obliquely within a sentence; wording within parentheses can be omitted without affecting the overall sense of the sentence. It may help to see parentheses as lessening the importance of the enclosed wording.
She sang (if you could call it singing) at the annual Garlic festival.
Tom Paine’s Common Sense (originally to be titled Plain Truth) was a masterpiece of political propaganda.
Do not punctuate around parentheses unless the punctuation would be required even without the parentheses. When punctuation is needed, it follows the second parenthesis.
The oatmeal had congealed into a cold, gloppy mess (it was the consistency of white paste), but the elderly man eagerly consumed a bowl and then asked for more.
As a general rule, do not use dashes or parentheses where you could not substitute a lighter punctuation.