DASHES and PARENTHESES are used in the same way.
They both act as SENTENCE INTERUPTERS.
However, they INTERUPT in different ways.
Here is a great GRAMMAR GIRL episode!
Grammar Girl – Dash, Parentheses, and Comma
What is a DASH?
A DASH is used when you want to draw alot of attention to the INTERUPTER.
You can use it to set off an APPOSITIVE.
Joe - the biggest kid in the class - was not going out for football this year.
You can use it when you want to REVEAL something, ONLY WHEN WHAT COMES BEFORE IT IS A COMPLETE SENTENCE.
There is only one thing I hate in this world more than seafood - raw seafood.
** Notice that there is a complete sentence before the DASH.
What are PARENTHESES?
PARENTHESES are used to interupt the sentence as well, but in a more subtle manner.
They are used to give commentary or your thoughts in a sentence.
She finally bought a car (a blue one with rust covering the trunk).
