En Dash –
Generally, the en dash indicates a range of numbers of a period of time. In this sense, it is used with numerals.

Examples
During the World War II years (1939–1945), basic commodities were rationed in many countries.
The school is open to children ages 6–14.
During office hours, 8 a.m.–3 p.m., someone will be here to help you with your application.

Em Dash —
The em dash (twice as long as the en dash) shows a big pause in a sentence or emphasizes the end of a sentence.

Examples
I’m ready to talk—if you’re ready to listen.
You said you’d—you said you’d be faithful to me!
Honda makes the best care in the subcompact category—the Fit.