They have different lengths and purposes.

There's the hyphen (-) you are used to then there's the en dash (–) and the em dash (—).

https://en.wikipedia.org/wiki/Dash

>An em dash (or an en dash) denotes a break in a sentence or to set off parenthetical statements.

>The en dash (but not the em dash) indicates spans or differentiation, where it may be considered to replace "and" or "to" (but not "to" in the phrase "from … to …")

>The em dash (but not the en dash) is also used to set off the sources of quotes

They also have ramifications for the systems that consume them. If a system is ascii and using extended ascii then globalization presents some specific challenges around these groovy usages of the seemingly innocuous little flat line.

Length (or width, if you think in terms of width and height). An en-dash is the width of a capital N; an em-dash is the width of a capital M. (Hence the names.)

They don't have the same width (more or less visible depending on the font): the hyphen (-) is for joining, the emdash (–) is for separating. The difference in width create a different "rhythm" in text. Yes, it's subtle, as it's usually the case for typography.