1. Indicates a sudden break or abrupt change in continuity: “If—if you'll just let me ex-plain—” the student stammered. And the problem—if there really is one—can then be solved.
  2. Sets apart an explanatory, a defining, or an emphatic phrase: Foods rich in protein—meat, fish, and eggs—should be eaten on a daily basis.  More important than winning the election, is governing the nation. That is the test of a political party—the acid, final test.—Adlai E. Stevenson
  3. Sets apart parenthetical matter: Wolsey, for all his faults—and he had many—was a great statesman, a man of natural dignity with a generous temperament…—Jasper Ridley
  4. Marks an unfinished sentence: “But if my bus is late—” he began.
  5. Sets off a summarizing phrase or clause: The vital measure of a newspaper is not its size but its spirit—that is its responsibility to report the news fully, accurately, and fairly.—Arthur H. Sulzberger
  6. Sets off the name of an author or source, as at the end of a quotation: A poet can survive everything but a misprint.—Oscar Wilde

See also: The Dash and the Hyphen.