An ellipsis--three spaced periods--is like a pothole in the road; something is left out, but that something doesn't really change the nature of the road. Ellipses show that words have been omitted from a quotation, words whose omission does not change the meaning of the quotation. Use an ellipsis when you don't need to use the whole quotation.
If the part of the quotation that you are leaving out occurs in the middle, use a space before and after the ellipsis:
According to Adrienne Rich, "A radical critique of literature . . . would take the work first of all as a critique of how we live, how we have been living, how we have been lead to imagine ourselves."
If the material you are leaving out occurs at the end of the sentence, follow the ellipsis with a period:
William Shakespeare wrote, "Love looks not with the eyes, but with the mind. . . ."
Tip: A full line of spaced periods means that a whole paragraph or one or more entire lines of poetry have been omitted.