...
The em dash is the longest of them all, and it’s commonly used to strongly offset a phrase. It should also be used for quote attribution.
Students on the Ithaca campus—especially those who have an interest in art—were delighted to hear about the renovations to the Johnson Museum.
"I love coming back for Reunion." —Jane Smith '10
The en dash is the second-longest, and it is used primarily to indicate ranges.
...