Several forms of dash exist:
The en dash (–) is used to indicate a a closed range of numbers ("see pages 96–122"), or a connection between two things of almost any kind: people ("the Nixon–Agnew administration", places "Papua–New Guinea), and so on. In these forms it often substitutes for the words to or and, and is always set without spaces on either side. En dashes are also used as a "super-hyphen" to link compound phrases that already include a hyphenated component.
The quotation dash is used to introduce quoted text.
Typewriters and computers have traditionally had only a limited character set, often having no key with which to produce a dash. In consequence, it became common to substitute the nearest incorrect punctuation mark or symbol. Em dashes are often represented by a pair of spaces surrounding a single minus sign (typical British usage) or by a pair of spaces surrounding two minus signs (mostly in the United States).
Modern computer software, however, typically has a much expanded character set and is usually perfectly capable of rendering both the en and em dashes correctly—albeit with a little inconvenience.
The HTML entity names for the em dash and the en dash are — and –. The equivalent HTML numeric character entity references are — and –. Nearly all web browsers and operating systems used today are capable of rendering the numeric form, and almost as many correctly display the named form.
In Unicode, the figure dash, en dash, em dash, and quotation dash correspond to characters U+2012, U+2013, U+2014, and U+2015, respectively.
Under Mac OS X, an em dash can be obtained in any application by typing shift-option-dash. An en dash can be obtained with option-dash.
In TeX, an em dash is typed as three hyphens ("---"), an en dash as two hyphens ("--"), and a hyphen as one hyphen ("-").
In professionally printed documents, the typographer sometimes adds a hair space on either side of an em dash (a refinement that is not practicable in electronic form given the limitations of current-generation web browsers) or even a full space, though this last is uncommon.
See also: meta:MediaWiki User's Guide: Creating special characters.Rendering dashes on computers