A Dash ( - ) allows a writer to introduce informality in writing or to add emphasis, by allowing a sudden change in thought or tone.
• Let us go to the market and do some shopping. I’ve run out of hankies and could do with a set of new-oops! I just recalled- today is Monday and the markets this side of the town are closed.
This sign indicates to readers that the speaker’s train of thought has been suddenly interrupted by something important that he’s just recalled.
In published writing, there are two kinds: the shorter one is called en-dash. And the longer one is called em-dash. Most word-processing programmes can distinguish between the two lengths. But in day-to-day writing no such distinction is made. Even many DTP (desk top publishing) houses and editorial persons do not know the difference.
In printing, the en-dash has certain specific uses (e.g., to indicate a range of numbers or dates, as in 1935037), whereas the em-dash is the one that is generally used to serve the purpose of regulation.
A pair of dashes is used to indicate asides and parentheses, which indicates a more distinct break than would be possible with commas:
• He was an excellent writer-although he had never gone to school-and had learnt the three R’s at home.
As a general rule, when the em-dash is used (as in the above example) there is no space between the dash and the words. However, if an en-dash is used, a single character space is left on either side. For instance, the same sentence would appear this way with en-dash.
• He was an excellent writer - although he had never gone to school - and had learnt the three R’s at home.
Although in many cases a colon could be used to punctuate a sentence and achieve the same effect, in general it is better to reserve the colon for formal writing.