A DASH indicates added emphasis, an interruption, or an abrupt change of thought. There are two types of dashes, the EM DASH, and the EN DASH. If you don’t know the difference, it’s okay!
An EM DASH sets apart parenthetical phrases or clauses in a sentence. In this way, “em dashes” are similar to commas and parentheses, but there are subtle differences.
An EM DASH can be thought of as a pause — with somewhat more emphasis than a comma and somewhat less than parentheses.
For example, “When the dress was finally delivered—nearly three months after it was ordered—she no longer needed it for the occasion for which it had been purchased.”
An EN DASH is wider than a hyphen (-) and narrower than an em dash (–). It has several uses:
An EN DASH stand in for “versus”; for example: Patriots–Rams Superbowl.
*An EN DASH shows range; for example: 90–100%
*An EN DASH is used to link the terminal points on a route; for example: Richmond–Chicago flight.
*AN EN DASH marks a partnership or pairing where both parts are equal; for example: U.S.–Canada Doughnut Alliance.
The dashes get their name from the width of the letters in their name. As an M is wider than an N, an EM dash is wider than and EN dash. The hyphen (not discussed here today) is the smallest.
Here’s how you can type them: http://www.computerhope.com/issues/ch001084.htm
Most style guides, except the AP, state that there should be no spaces between an em dash and the adjacent words. Thus, if you are writing for a newspaper, magazine, or website that uses the AP style guide, put in the spaces. If you are using a style guide that advises against the spaces, leave them out. If you are not under a style guide when writing, then it is your choice — but whichever style you choose, be consistent throughout the body of writing.