The Mysterious Em
I certainly can, Marilyn. I'm glad you asked.
Basically, there are three kinds of dashes in common use. The choice of which one to use has to do with the way the words are connected (or not). Here they are:
Hyphen: - En Dash: -- Em Dash: ---
Each of these little fellas has a job, and each of those jobs are distinct. When you understand their differences, choosing which one to use becomes a piece of cake. Kind of ...
The Hyphen. Hard hyphens are used to join compound words that have not yet metamorphosed into permanent compound words. Words like dim-wit, and terms like foot-and-mouth disease feature hard hyphens. Often, they are used to attach a prefix or suffix to a word to make a "one time only" compound word (he was a warthog-faced buffoon, or she had alligator-like skin).
The En Dash. An en dash is a strange little creature you've doubtless seen many times, but just have not noticed. It is typically used in three situations: (1) when one element of a compound is already hyphenated and you want to add a prefix or something else to it, (2) as a substitute for the word through in a range, and (3) to report scores or tallies.
The Em Dash: The em dash is the real name for the mark most people refer to as a "dash." It is used to indicate an interruption in thought or speech. It can also be used to join independent clauses (an independent clause is part of a compound sentence that could be hacked off from the rest of the sentence and still make sense on its own). By convention, there is no space before or after an em dash.
Here are some em dash examples:
I love to eat ice cream---especially vanilla and rocky road---but only in a cone.
"Jim," Vanessa said, groping around on her desk. "Have you seen my glasses? I swear I left them right---"
I like goldfish and cats---dogs are too feisty for my tastes.
There are instances in which you would attach two (or more) em dashes together (they can work to replace part of an expletive, for example), and there are other uses for hyphens, too (such as in suspended compounds) but these are the most common uses of our friend, Mr. Dash.