I ate six slices of pizza and got a stomach-ache.
An em dash, though, sets apart elements of a sentence that explain something, similar to a comma or a pair of parentheses:
The traveling vacuum cleaner salesman—always a nuisance to neighborhood residents—hadn't been seen in several weeks.
The hyphen ( - ) is short, while the em dash ( — ) is much longer. The two look different on the printed page and shouldn't be confused, though confusion often arises because there's no em dash key on a standard keyboard. Many writers will type hyphens or double hyphens in place of em dashes, which look silly and unprofessional.
Here's how to type an em dash using Microsoft Word:
1. Type out the text before the em dash, without a space.
2. Press the hyphen key twice.
3. Again, without using a space, type the word you want to follow the em dash.
4. Now, hit Space. Word will automatically substitute the em dash for you.
Alternately, Windows users can access the em dash using the Character Map, that long list of special characters like ©, ¥, Ω, ∆, and ♫, to name but a few. To access the Character Map, click the Start menu, then All Programs, then Accessories, then System Tools, and finally, Character Map. You'll find the em dash near the end next to its seldom-used cousin, the en dash, and its special character cousin the horizontal bar.
Once you've located the em dash, click Select to pull it into the box at the bottom of the window, then Copy to add it to the clipboard. You can then paste it into MS Word and other programs as needed.
Writers: use proper em dash formatting whenever possible! It makes you look more professional, it's easier on the eyes, and it saves your editors manuscript cleanup time down the road. You should also know the word processing tools of your trade the same way carpenters know their hammers, biologists know their microscopes, and golfers know their clubs. There's no reason not to.