There are two ways to end a line of dialogue that isn’t meant to stand as a complete sentence. One is with a dash, the other is with ellipses (those three spaced dots, remember?)
They aren’t interchangeable.
Ellipses are for utterances that trail off in some manner:
“Well,” she said, “if that’s what you really want . . . .” (That’s the ellipses, plus a period.)
“Well . . . if that’s what you really want, I suppose it’ll have to do.” (That’s just the ellipses, showing how the speaker lets his or her voice trail off into a significant pause before going on to the rest of the sentence.
Dashes are for utterances that are broken off or are interrupted:
“I told you I wanted–”
“I know what you told me, but the store was all out of them.”
Or:
“And the winner is–”
(Drum roll.)
“Anastasia Oddfellow of East Drumstick, New Jersey!”
Got it? Good.
Dr. Doyle's Blog 
This comes in handy for me right now. Good to get the nuances of these two bits of punctuation straight. Thanks.
One ellipsis, two ellipses.
I stand (well, actually, sit) corrected.
It’s one of those pesky words with plural items, like those dots, lurking inside a singular noun — and one that ends in -s, to boot. The mind will keep trying to make it plural.
Which is why I’m a line-editor and a content editor, not a copyeditor. (Spell-check keeps trying to make “copyeditor” into two words, but one of the best copyeditors I know maintains that it is one word, no hyphen, and who am I to argue?)
Er, TexAnne, I mean. Stupid “correct a typo, make a typo” law of nature.
Yep. Gets me all the time, too.
I comfort myself with the thought that scribal errors have a long and (in)glorious history.