DASH

The dash (two hyphens on the computer, no space in between) is
used for emphasis or a sudden interruption in thought. The dash
should be used sparingly because the words enclosed in the dashes
are strongly emphasized.

        Our district manager--fully aware of the shortage--shipped you a replacement
        immediately.

        They started the project last week--but this is not your concern.

HYPHEN

Use a hyphen to tie two or more words together when they describe
the word immediately following.

                    first-class mail                    speed-building device

                    city-wide survey                  up-to-date report

Use a hyphen to join a numbered figure and its unit of measurement.

                    40-hour week                     13-cent stamp

                    30-pound turkey                 10-day trip

Certain expressions are spelled with a hyphen.

                    ex-senator                           secretary-treasurer                 President-elect