An en-dash is less wide than an em-dash. An em-dash is used as a dash, that is, in parenthetical situations. The en-dash is used pretty much exclusively for ranges. They are both wider than the hyphen, which is used in hyphenated words. The em-dash and en-dash are supposedly named that way because the em-dash is as wide as a capital `M' and the en-dash is as wide as a capital `N', but that's rarely actually true anymore.