DASH --

The dash is represented by two hyphens with no space 
around them (--).
A dash may be used:

To separate two clauses when the second amplifies or 
restates the first
Example:
The shirt was very cheap-- the cost was ten dollars. 
 When the enclosed element contains internal commas
Example:
The devices--processor, modem, hard disk, and cards--are 
the essential elements in this computer.
Before a summary of preceding details in a sentence
Example:
 
Processor, hard disk, and modem--These are the typical devices 
he needs to change.
To separates a group of antecedents from their pronoun 
that is the subject of the sentence.
Example:
Car, plane, train--these are the possible means of 
transportation in this town.
To enclose elements those need emphasis.
Example:
We are--human being--greedy.
To show a sudden change in structure of a sentence
Example:
 
He failed the exam--the final rectification will be held 
soon--which the first question was about how to know 
without learning. 
Make an appositive more emphatic
Example:
 
There is a simply way to have a success-- the knowledge.
 Enclose a complete sentence that interrupts another
Example:
The car that we have seen yesterday costs a lot -is made in 
Germany.

In this case, (- is an en dash, not a hyphen. A dash is called 
also an em dash)

Commas are insufficient to enclose an interrupting sentence; 
dashes or parentheses are necessary.
To connect a unit modifier with a two-word or hyphenated elements
Example:
Saint John--Vancouver distance
Fifteen-years-old--mother-in-low relationship
 To separate, in list, the item from a descriptive statement
Example:
They were supposed to bring:
1- Flour--Four pounds in one box.
2- Milk--The fresh one.
3- Eggs--Big, small and medium.
 To Indicate inclusive or continuing numbers or dates
Example:
Read all the pages: 10--19
Sept--Apr.

But it is not correct with from . . . to . . . or a 
between . . . and . . . construction.
 To show vague or unlimited dates
Example:
1945--
600 to 199--
To separate a title and subtitle
Example:
Prepositions with time--How to use preposition-- have 
been understood.
To emphasize an explanation (more than a comma or a 
semicolon) with the words: namely, that is and for example
Example:
They are many strong words in her email--for example, I hate 
to be discovered. 

NOTE:

 Commas indicate only a little separation within the sentence.
 Dashes emphasize the element enclosed and clarify its meaning 
when It contains internal commas.
Parentheses indicate that the enclosed element is only freely 
connected to the rest of the sentence. 
The colon or semicolon may also be used for this purpose. 
The dash is more emphatic than the semicolon.