From FontShop founder Erik Spiekermann
There are three different ways to connect or separate words: the hyphen -, the en dash –, a little wider than the hyphen, and the em dash —, wider still. The regular hyphen is easily accessible on any Mac or PC keyboard, whereas the en dash needs the combination option-hyphen on the Mac. The em dash is accessed by pressing option-shift-hyphen on the Mac. The use of these dashes depends on house styles and tradition. The em dash with no space around it is traditionally used to separate thoughts—like this one—but I think its length is a distraction in running text. Try using the en dash to separate thoughts – like this one – with a character space on either side. En dashes without space on either side are also used between numbers and compound words as in: the shop is open 10–7, while you can take the New York–Kansas City train or the New York–Baltimore train only 8am–3pm.
This is a good, if overly-simple, description of usage from Washington State University. And if that wasn't simple enough for you keep this in mind “Dashes tend to separate elements and hyphens to link them.”