A hyphen is used to join things, such as compound modifiers when an adjective and a noun combine to modify another noun: small-claims court, full-court press, three-hour opera.
But we do not use hyphens when, as is often the case in bureaucratic or corporate writing, noun modifies noun modifies noun: bandwidth functionality synergy or similarly meaningless terms. We also omit the hyphen when the adjective-noun compound has become a widely recognized unit: high school student, civil rights case.
The Associated Press Stylebook caused a low-grade hullabaloo recently by saying its editors are OK with omitting the hyphen in compounds such as third grade student. Insert or omit as you like, because the AP Stylebook is not the boss of you.
This is a dash: —
It is called an em dash because it is as wide as the letter M. The em dash is used to separate things, such as when a new thought breaks continuity—supposing you’re actually thinking.
Journalists are particularly susceptible to overusing the em dash, diminishing its effectiveness, merely to set off parenthetical phrases.
If you were trained to operate a keyboard on a typewriter, you may be accustomed to using two hyphens, --, to indicate a dash.
Stop doing that.
Find where the em dash lurks in your word processing software, under “symbols” or “special characters,” and start using it properly.
Some of you, I notice, use a single hyphen in place of a dash.
Stop doing that.
There is another dash, the en dash, the width of the letter N, that is used, for example, to separate the elements in a range: the years 1960–1969. The en dash is used mainly in book and journal publishing, so you may not have much call for it.