What is an ellipsis? To be honest, I was going to come up with something quirky, but I am angry with my sister right now. So angry, in fact, that I....
See what I did there? Ha! And that was unintentional. I was really going to tell you that I am so angry with her that I actually had a dream last night in which I was yelling at her. But instead of finishing my sentence, I trailed off with an ellipsis. You will noticed I used four periods. An ellipsis is a series of three dots, but because I ended the sentence with it, I still have to add the period to signify the close of the sentence.
The most common usage of an ellipsis, is during dialog. In fiction writing, (in non fiction, an ellipsis indicates the omission of a word or words) we all know how important it is that our characters are individuals. A big part of characterization, especially in writing YA, is dialog. I've seen some writers using an ellipsis to signify an interruption, or to separate a clause, and this is incorrect. In such cases, you should use and em dash.
Em dash from Dictionary.reference.com:
em·dash or em dash (ěm'dāsh') n. A symbol ( — ) used in writing and printing to indicate a break in thought or sentence structure, to introduce a phrase added for emphasis, definition, or explanation, or to separate two clauses.
Just so we're clear, I'm going to give an example of each.
Em Dash:
"James, I dunno what to tell you. I think--"
"Don't think. Just don't."
James interrupted the first speaker. He cut off her sentence so we used an em dash.
Ellipsis:
"James, I think...I dunno what I think."
The speaker did not complete her thought, so I used an ellipsis to signify that.
I realize these are both poor examples, but you get the idea.
Having said all of this, ellipsis can be wonderful tools in writing dialog, but over use can make your writing hard to read, choppy and just down right poor. If your sentence ends in a full thought, one dot will do.
