Em dashes, En dashes, Horizontal Bars and Hyphens

An em dash (U+2014) is the longest.  It is the width of an upper case M. In Windows, it can be created with [Alt+0151], and in HTML with [ & mdash;].  It can be used in place of parenthesis or a colon or it can represent an abrupt change of thought. Example:

Red, white, and blue—these are the colors of the flag.

An en dash (U+2013) is half the width of an em dash. In Windows, it can be created with [Alt+0150].  It is used to indicate a range of numbers (although another source says that it may not be used to replace the word "to" or "through").  It is also used to indicate a paired relationship. Example:

For ages 3–5.  McCain–Feingold.
“The New York–Boston line"

A hyphen (U+2010) is used to make compound words.  Example: "a man-eating shark".

A horizontal bar (U+2015), also known as a quotation bar, is used to indicate quoted text.  It looks exactly like an em dash, but technically it is a different character. Example:  "―Oh Miss Douce! Miss Kennedy protested. You horrid thing!"

A minus sign (U+2212) is the same as a hyphen in ASCII, but technically it is a different character.

A hypen-minus (ASCII 45, U+002D) is used in ASCII where it is not necessary to distinguish between en dashes, hyphens, and minus signs.

A double hyphen-minus (--) is used in ASCII to represent an em dash.