Colons can be used to join two independent clauses when you wish to emphasize the second clause.
e.g. I forgot my umbrella: we got soaked.
Colons can be used to introduce a clause that reflects something in the clause preceding the colon.
e.g. We like several types of movies: romance movies are emotionally moving, but action movies are really exciting.
Colons can be used to introduce lists, quotations, and explanations.
e.g. I have many different favourite teams: the Oilers, the Flames, and the Blue Jays.
e.g. U.S. President George Washington once said the following: "To be prepared for war is one of the most effectual means of preserving peace."
e.g. This is how it happened: I had a quarter for the meter, then I dropped it down the sewer crack, I come out of the store and suddenly, I have a parking ticket!