The dash is a punctuation mark that is similar in appearance to the hyphen and minus sign, but differs from these symbols in both length and height. The most common versions of the dash are the en dash (–), longer than a hyphen; the em dash (—), longer than the en dash; and the horizontal bar (―), whose length varies across typefaces.
This is a companion discussion topic for the original entry at http://grammarerror.com/tags/dash-and-hyphen