The Dash is a longer line than a hyphen. It is used when a repetition or explanation or qualification of a previous statement is made.

I can do no more to comfort you than to tell you that he died as he lived, a brave, true man — the best of comrades and staunchest of friends.

The dash is also used to indicate a break in thought, or to introduce some important words.

Have you read — but, what I am most interested about, is your sister with you, and is she well and happy?
Remember what Simonides said — that he was never sorry that he had held his tongue, but often that he had spoken.

Sometimes dashes are used instead of brackets.

My friend — the one I told you about yesterday — will be here tomorrow.