There are three types of dashes or lines used in writing. Each differs in usage yet serves a particular purpose in sentence structure that cannot be ignored.
The hyphen and en dash are relatively the same size and are located to the right of the zero on most standard U.S. keyboard layouts. The em dash is a longer dash, about the length of two en dashes.
The hyphen is used to combine two compound words together or to connect non-continuing numbers such as telephone numbers.
The en dash is used to connect continuing numbers, dates, times, etc.
The em dash is used to show a sudden break in thought that disrupts the sentence structure.
The en dash, or "n" dash, is called so because it was traditionally the length of an "n" while the em dash, or "m" dash, was the length of an "m" on a typewriter.
Many grammarians follow the belief that the em dash is used too freely and that instead the semi-colon should be more carefully considered when writing.
Most word processors allow the creation of the em dash by putting two en dashes together. If your word processor doesn't support this, then you will have to add the character under a special characters dialog.