In pairs, dashes have the same grammatical function as parentheses: they isolate a group of words that the sentence can function without. While parentheses indicate that something isn't particularly important, though, dashes emphasize whatever goes between them.
Appositives (nouns) and modifiers (adjectives)—in series or alone—work well between dashes.
In general, the appositives or modifiers between the dashes describe the noun (or noun phrase) directly before the dashes.
There should be no space before or after the dash.
In these examples, the appositive phrases are in bold and the noun (or noun phrase) they apply to is italicized.
The witch in "Hansel and Gretel"—an archetypal fairytale villain—shows no sympathy towards the two children who begin eating her house.
I lay helplessly in the snowbank and watched as everyone else in sight—skiers, snowboarders, a woman wearing only one ski, a tiny child on a miniature snowboard—flew down the mountain with no trouble at all.
In these examples, the modifiers are in bold and the noun (or noun phrase) they apply to is italicized.
The witch—wicked, heartless, conniving—shows no sympathy towards the two children who begin eating her house.
My first day on a snowboard, I think I fell every way possible—backwards, forwards, on my head, into a snowbank—until I had nearly convinced myself that winter sports just weren’t for me.
Dashes can be a great way to integrate multiple short quotes that all support a claim you're making. Make sure you use parallel structure when you do this: all quotes should function as the same part of speech. In this example, the quotes are all noun phrases.
Shelley describes the summer Victor spends working on his project with tranquil phrases — "most beautiful season"; "plentiful harvest"; "luxuriant vintage"; "charms of nature" — setting up a contrast against the confinement of Victor's isolated laboratory (41).