Em dash

A dash that is of the width of one 'm'.

HTML — &mdash or &#8212
Chicago — 6.87
Wikipedia — http://en.wikipedia.org/wiki/Em_dash#Em_dash

Usage:
Has several different kinds of uses. Basically used to set off a section in a sentence that has an independent meaning or a 'life of its own' and adds some meaning to the rest of the sentence. It does the task that can normally be done by a comma, parenthesis or a colon.

(I'm not giving any examples here coz the Internet is full of examples.)

As an editor and a writer, I hate the current hegemony of the em dash in books and newspapers. It is almost vulgar the way newspaper editors all over the world are resorting to the em dash instead of investing some intelligence to use the correct punctuation. When you have perfectly good grammatical tools for the purpose, why use em dashes? Em dashes are taking over commas and colons, and in some cases, over ellipses and semi-colons. Also, I hate not having single spaces on both sides of an em dash.

Em dash is a beautiful dash. Lets use it, not abuse it. I believe that an em dash is best used for major and impactful setting-off, rather than minor or customary setting-off, which can be taken care of by commas and colons and the like.

One of the better definitions is given by this website — "...is used to show a break in thought or a shift in tone."