Ah, the colon. After the comma and the semicolon, the colon is probably the most misused punctuation mark we have. Today, I present a quick and dirty lesson on this strange little mark.
Colons are generally used in order to introduce a list. Thus, if you want to tell what items the evil sorcerer carries in his man bag, you would write,
Enfido always carried the following necessities: his grimoire, a vial of fast-acting poison, and a bottle of uisce.
What you should also notice about the above sentence is that I did not use the colon to separate the verb (carried) from its object (necessities). The clause preceding a colon must always, always be a complete sentence. Thus, though I might have wanted to shorten the sentence to read, Enfido always carried: his grimoire, a vial of fast-acting poison, and a bottle of uisce, this would be incorrect, leaving carried without an object.
A colon can also be used to introduce a clause, phrase, or sentence that summarizes the preceding clause. For example, if Enfido recognizes his old nemesis Bawdler despite his disguise, I might write,
Enfido saw through the beggar’s disguise: the “beggar” bore the tattoo of the Brotherhood on his inner wrist, just as Bawdler did.
In the above sentence, the colon introduces how Enfido recognizes his old brother-in-arms, thereby summarizing the first clause.
That’s it! Those are the two uses of the colon. Not so mysterious after all, is it?