Let's talk about the em dash. This dash is the widest of the dashes and is called em because it is about the width of the capital letter M. It can be used to add emphasis, to show an interruption, or an abrupt change of thought and it can be used in place of commas, parentheses and semicolons. In my humble opinion, it adds versatility to your writing.
You create an em dash on most computers by typing the first word, holding down the ALT key and then entering 0151 on your numeric key pad. You then enter the next word with no spaces between.
Here are a few examples:
"Come on, I told you we won't get cau—Oh, hey, Mom." (Interruption)
"I go crazy over dark chocolate—well, actually, any kind of chocolate—when I'm sad." (parenthetical)
"here is only one thing missing from your plot—a great-looking guy." (emphasis)
"Let's grab some lunch—whoa, did you see the size of that dog?" (abrupt change of thought)
This is by no means an exhaustive list of ways to use this dash; go to punctuationmadesimple.com for more. But a word of caution: Overuse of this dash can be as irritating as the overuse of an exclamation point, so use it sparingly.