I learned something at work this week that I can't believe I didn't pick up during my years as an English teacher—the difference between hyphens, en dashes and em dashes. I guess being a web developer can lead just about anywhere.
Hyphens are short or just "normal" dashes. Any standard keyboard will have a key for them. Hyphens are used for joined words.
En dashes are longer than hyphens. If you have a Mac you can type them by hitting option+dash. En dashes are generally used for ranges.
Em dashes are very long compared to hyphens. Long ago, typographers named en dashes and em dashes after the widths of the letters N and M. On a Mac you can type em dashes with option+shift+dash. Em dashes can sometimes replace a comma, semi-colon or colon—right here for example. They can also replace parenthesis in parenthetical asides such as the following:
"My great-grandmother—a life-long pianist—used to enjoy that piece."