—

The em dash is the dash of prose: it opens dialogue, sets off the aside, marks the interruption. It is as wide as an M — hence the name — and it is the largest gem in the drawer.

The common mistake is the doubled hyphen --, a typewriter inheritance that word processors fix in silence. On the web nobody fixes it for you: it's — or the character itself.

Wrong:  The web - said the manifesto - would belong to everyone.
Right:  The web — said the manifesto — would belong to everyone.