Long dash (–)
A long dash, or em dash, is used:
in pairs, to mark off information or ideas that are not essential to an understanding of the rest of the sentence:
Thousands of children—like the girl in this photograph—have been left homeless.
My son—where has he gone?—would like to meet you.
to show other kinds of break in a sentence where a comma, semicolon, or colon would be traditionally used:
One thing’s for sure—he doesn’t want to face the truth.
Things have changed a lot in the last year—mainly for the better.
Dashes are especially common in informal writing, such as personal e-mails or blogs, but it’s best to use them sparingly when you are writing formally.