The em-dash is used to indicate a "pause" in a sentence. Eg, from the book I'm working on at the moment:
Quote:
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Their eyes met, and something—something unspoken but cogent—passed between them.
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The hyphen is simply used to join loosely-connected words (as I've just used it there), or to break a word at the end of a line.
Hyphens and dashes are gramatically completely different things.