Hyphens and Dashes

When should you use a hyphen? What's the difference between a hyphen and a dash? When should you use a dash?
The most effective way I could find to explain the uses to new reporters was this: a hyphen joins, a dash separates.

A hyphen (-) joins. It has no space before or after it. It is used with compound nouns and with two-word adjectives preceding nouns as well as other uses.
Examples:
Dec. 12-17
rip-off
the big-headed writer
the score was 28-14
a 2-year-old filly
All of the 3- and 4-year-olds

A dash (—) separates. It has a space at both ends. It can show an abrupt change. It can function like a hard comma. Some editors use dashes only in pairs.
Examples:
The reporters — Bill, Shelley and Matt — turned in their stories.
The commission raised the mayor's salary — his first since assuming office 10 years ago — over the objections of the city manager.

An ellipsis (...) is NOT interchangeable with a dash. It is used in place of words deleted from a quote or document, usually to eliminate unnecessary verbage. It has a space before and after.
Examples:
I pledge allegiance to the flag ... and to the Republic for which it stands ...