A 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 emails or blogs, but it’s best to use them sparingly when you are writing formally.