Dashes
Use dashes to emphasize or set apart comments in your sentences. A pair of dashes functions like parentheses. A single dash functions like a colon. The dash, however, is less formal and more dramatic.
Never use a hyphen (-) in place of a dash (—). It’s just wrong and looks incredibly wimpy.
- INCORRECT: This is important – are you taking notes?
- CORRECT: This is important — are you taking notes?
Hyphens
Use hyphens for compound words and to hyphenated words at the end of a line. For example: T-shirt, well-known, sixty-five.
Colons
Use a colon to introduce a related idea or items within a sentence. For example, Platt students have a choice of the following disciplines: video, 3D animation, web, and print design.
If the sentence has two independent clauses that are related, separate the independent clauses with a semicolon. For example, Platt students have many choices; they can study video, 3D, web or print design.
For more information, refer to Grammar Girl, “Dashes vs. Colons“.