Dashes, Parentheses, Brackets, Ellipses
The following punctuation marks should be used sparingly, as they are more specialized than those that appear above.
Use a dash to draw attention to parenthetical information, to prepare for changes in tone, or to introduce or emphasize information.
CORRECT: Everything about the test - especially the surprise essay section - was incredibly difficult.
CORRECT: He took a deep breath, began to sprint toward the finish line - and collapsed just a few feet before he crossed it.
CORRECT: I need three things from you - patience, kindness, and understanding.
CORRECT: I hate - absolutely abhor - that band.
Dashes are fairly versatile, so there aren't many obvious errors related to their usage. Just be careful not to overuse the dash, as it's less formal than other punctuation. Also remember that a dash is composed of two lines (--), not just one (-), which is a hyphen9.
Use parentheses to set off extra material, digressions, or afterthoughts.
CORRECT: After her eggs, bacon, and coffee (her usual breakfast), she leaves for work.
CORRECT: My mother tolerates (i.e. hates) my nose piercing.
Brackets are used to enclose words that you add to a direct quotation10.
CORRECT: In Slaughterhouse-Five, " he [Billy] is in a constant state of stage fright, he says, because he never knows what part of his life he is going to have to act in next."
If there is an error in the original quotation, the word sic appears in brackets after the word containing the error.
CORRECT: "This sentence contains an eror [sic]."
Paraphrasing quotations can eliminate the need for this use of brackets. Try paraphrasing to avoid calling attention to others' mistakes whenever possible.
The ellipsis is used to show the deletion of words from a direct quotation.
If the original quotation says:
ORIGINAL: "The best way to be healthy, according to the most prestigious doctors at the University of Pennsylvania School of Medicine, is to eat right, exercise, and get plenty of sleep."
You can use the ellipsis as follows:
REPHRASED: "The best way to be healthy...is to eat right, exercise, and get plenty of good sleep."
Keep in mind that you don't need to use the ellipsis to show words omitted at the beginning of a quote. If you paraphrased the beginning of the quote above, you wouldn't need to use ellipsis:
REPHRASED: If a person wants to be the healthiest he can, he should "eat right, exercise, and get plenty of good sleep."
You also don't need to use the ellipsis at the end of a quote unless you are omitting words from the end of a multi-sentence quote. Use brackets around the ellipsis in this case, to show that the mark itself is not a part of the original sentence.
If the original quote reads:
ORIGINAL: "The best way to be healthy, according to the most prestigious doctors at the University of Pennsylvania School of Medicine, is to eat right, exercise, and get plenty of sleep. This is the first of many important suggestions, the rest of which deal with managing everyday pressures such as family, career, and other commitments."
You could abbreviate it as follows:
REPHRASED: "The best way to be healthy, according to the most prestigious doctors at the University of Pennsylvania School of Medicine, is to eat right, exercise, and get plenty of sleep. This is the first of many important suggestions[...]."
9 Hyphens are most commonly used within single words (i.e. to connect compound words, as in well-known, or to attach prefixes or suffixes, as in self-respect).
10 Words are often added for clarity, especially if the quotation is heavy with pronouns. See Hacker for examples.