What are em and en dashes?

EM DASH

Used to denote an abrupt change in thought in a sentence or an emphatic pause. Always has a space on both sides, except when used at the beginning of a paragraph.

e.g. “I’m going to Hawaii — if I get a raise.”
e.g. “Charles had a birthday — his 40th — on May 12.”

Used when a phrase that otherwise would be set off by commas contains a series that must be separated by commas. Always has a space on both sides, except when used at the beginning of a paragraph.   

e.g. “We have everything we need — hamburgers, buns, chips, cheese, soft drinks — for tonight’s cookout.”

EN DASH

Used in place of the word “to” or “through,” or to connect two nouns of equal importance. No spaces on either side.

e.g. “Please refer to pages 37–45 in your textbook.”
e.g. “The New York–Chicago flight has been delayed.”