Dashes -- are rarely used in formal writing and should be used sparingly.  They are primarily used to indicate a pause in thought which offers more detail or emphasizes a point of information.

• I love going to the theater -- movies, that is.

They can be used to separate an appositive phrase, usually a list, from the rest of the sentence.

• I would always read classic science fiction -- Herbert, Asimov, and Heinlein -- due to its contemporary social commentary.