Way back when type was actually set, there was an "em" dash and an "en" dash. The em was the dash, and the en was the hyphen. They have two different functions. The former is a mark of punctuation. The latter is used to hyphenate, separate parts such as a phone number, or as a minus sign. (When we used to type, we would key two hyphens side by side to signify a dash.)

If you type a hyphen and space, Word usually changes the "en" into an "em." If you don't space, you get the smaller version. Be careful in your documents. Make sure that your hyphens are hyphens, and your dashes are dashes.

(And if you play Scrabble, en and em are easy ways to get a few extra points.)