Colons tell the reader that what comes next is directly related to what preceded it. A colon always follows an independent clause (complete sentence). Colons are often used to list things, but a clarification can also follow a colon. You may write complete or incomplete statements after a colon.
Example: My English teacher has two favorite foods: pasta and ice cream.
In rare cases, you can reverse the normal order of your clauses for a different effect.
Example: Wealth, fame, power: One man had acquired all of these worldly things.
Semicolons separate two related independent clauses (complete sentences).
Example: I hope to spend a great deal of time reading on vacation; I just purchased the latest Stephen King novel.
There is one other use for semicolons. Normally, we use commas to separate items in a list. Sometimes, however, each item in a list will itself contain commas. In this situation, commas do not clearly communicate where one list item ends and another begins. Use semicolons instead of commas in these situations.
Example: I brought three friends on my journey: Jeff, the tough guy; Annie, the brain of the group; and Shirley, who can bake a mean pie.
Dashes set up a break in the sentence or to separate a long appositive from the rest of the sentence.
Example: The bank teller - who incidentally also works at Starbucks on the weekends - does not like to drink coffee.
* An appositive is a phrase that either renames the noun or the phrase right before it. For example: My teacher, an expert in Middle Eastern politics, gives us many quizzes. The appositive - "an expert in Middle Eastern politics" - is not necessary to the general content sentence, but does provide extra information.