This is the punctuation mark known to grammarians as a dash, and to typographers as an em dash. It is used in English to indicate a break in the flow of a sentence — as Strunk and White put it, an interruption "stronger than a comma, less formal than a colon, and more relaxed than parentheses."

A pair of dashes, like a pair of parentheses or commas, can delimit a parenthetical comment or aside. However — unlike parentheses — dashes emphasize the delimited phrase rather than diminishing it. In academic styles where parentheses indicate a bibliographic reference and square brackets (a sometime replacement for parentheses) indicate alteration of quoted text, dashes may be the only acceptable substitute.

In dialogue, a dash at the end of a quoted sentence fragment may indicate that another person has interrupted the speaker, just as an ellipsis would indicate that the speaker has trailed off.

John said, "Let us eat, drink, and be merry, for tomorrow —"

"Don't say it!" exclaimed Mark. "I won't have any more of your fatalism!"

ASCII does not contain a dash character. A double minus sign or hyphen is the most common substitute -- just like that. This substitute is unnecessary in HTML — you can write an em dash using the character reference —.