The dash is the interrupter.  Writers use the dash as follows:

• To set off independent clauses inserted inside other independent clauses.

  Samuel Robert Williams—his friends call him Wizard—runs the most progressive newspaper in Wisconsin.

• To set off lists inserted in independent clauses.

  Women tolerate qualities in a lover—moodiness, selfishness, unreliability, brutality—that they would never countenance in a husband. Susan Sontag

• When, after a list, a sentence starts again with a summarizing all, these, or those.

  The visual essay, the rhythmic album, the invitation to drop in on a casual conversation—these are the idiosyncratic traits by which television, as television, has come to be recognized. Walter Kerr

• To isolate a word or phrase for emphasis or for climactic or humorous effect.

  I could never learn to like her—except on a raft at sea with no other provisions in sight. Mark Twain

• When writing dialogue, to indicate a break in speech or thought.

  "How are you?"
  "I'm—uh—well—I guess I'll live."

Typographical Notes
Do not space before or after a dash. To make a dash, use two hyphens and refrain from spacing before, after, and between those hyphens.  Please note that most word processors will automatically replace the -- with a proper dash.

The Dash with Other Punctuation
Do not use other punctuation (comma, semicolon, colon, period) in conjunction with the dash. The only exceptions to this rule are the exclamation point, the question mark, and the quotation mark.
 

• Clause: a group of closely related words containing a subject and a finite verb.
  • Independent Clause: a clause that makes sense as a sentence in itself.
    • Susan loves her English class.
  • Dependent Clause: an independent clause plus a subordinating conjunction.
    • although she is nervous about her first paper
• Subordinating Conjunctions: after as, although, as if, as long as, because, before, even if, even though, if, if only, in order that, now that, once, provided, rather than, since, so that, than, that, though, 'till, unless, until, when, whenever, where, whereas, whether, while
• Coordinating Conjunctions: for, and, nor, but, or, yet, so (remember "FANBOYS")
• Conjunctive Adverbs: consequently, furthermore, however, moreover, nevertheless, otherwise, then, therefore, thus, furthermore, similarly, etc.

Updated: 08.18.07