Punctuation rule on dashes

Dashes--use a pair of dashes, not just one, to mark the beginning and end of a word group that needs emphasis. An em dash is the width of an m. Use an m dash sparingly in formal writing. In informal writing, em dashes may replace commas, semicolons, colons, and parentheses to indicate added emphasis, an interruption or an abrupt chance of thought. You can find this here http://www.grammarbook.com/puncuation/dashes.asp