A dash is a long horizontal mark twice the length of a hyphen. On most typewriters and computers dashes are represented by typing two hyphens.
Dashes are emphatic. They are nearly like emphatic parentheses.
To be effective, dashes, like exclamation points, should not be overused.
Dashes indicate an abrupt change of thought.
Sometimes they set off a clause or phrase for emphasis or dramatic effect.
Change of thought: I loved the dinner last night--have you you ever been to Chez Louis?
Set off statement for emphasis: Punctuation marks can be confusing--commas, dashes, hyphens, colons!
See also Parentheses and Dashes with Nonrestrictive Modifiers.