The ellipsis (plural ellipses) is the mark that indicates the omission of quoted material, as in "Brevity is . . . wit" (stolen shamelessly from an episode of The Simpsons). Note two things: first, most typing manuals and house styles prefer the periods to be spaced, thus:
Brevity is . . . wit.
(In electronic communication it's sometimes convenient, even necessary, to run them together, since line-wrap can be unpredictable.) Second, and more important, is the number of periods. The ellipsis itself is three periods (always); it can appear next to other punctuation, including an end-of-sentence period (resulting in four periods). Use four only when the words on either side of the ellipsis make full sentences. You should never use fewer than three or more than four periods, with only a single exception: when entire lines of poetry are omitted in a block quotation, it's a common practice to replace them with a full line of spaced periods.
One other thing. Although it's a matter of house style, note that it's usually unnecessary to have ellipses at the beginning or end of a quotation; they're essential only when something's omitted in the middle. There's no need for ". . . this . . ." when "this" will do: readers will understand you're not quoting everything the source ever said, and that there will be material before and after the quotation you give. The only time it's advisable is when the bit you're quoting isn't grammatical when it's standing on its own: "When I was a boy . . ." — that sort of thing.
WRITING LESSON: ELLIPSES
