Punctuation ... --

Here are two elements of punctuation that are helpful in writing.

Ellipsis (…) is used in non-fiction to show where something has been left out of a quoted text. “Hey diddle diddle, the cat and the fiddle, the cow jumped over the moon” becomes “Hey diddle diddle, the cat…the moon.”
The ellipsis shows a reader something has been omitted from this sentence.

In a longer passage, the complete sentence is followed by a period and the ellipsis goes before the next line of text. “Hey diddle diddle, the cat and the fiddle, the cow jumped over the moon.…the dish ran away with the spoon."

In fiction the ellipsis can be used to represent hesitant dialogue. “You shouldn’t…you shouldn’t be doing that.”
Slate has an interesting article that examines this more.

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Dash (—), called an em dash in printer’s language, is often represented by two hyphen marks (--); many word processors automatically change this to an em dash. The em dash is most often used to set off a parenthetical clause. I could have written

"The dash—called an em dash in printer’s language—is represented by…"

In dialogue the em dash represents interrupted speech.

“You shouldn’t be doing tha—”
“Why not?” his friend demanded.

When an em dash is used to show interruption, no punctuation follows it.

No space should precede or follow an ellipsis or an em dash.