The colon ( : ) is used to
a) introduce lists. In this case, the words in front of the colon must be a complete sentence.
Correct example: My new car came equipped with many options: power windows, power locks, a sunroof, and a CD player.
Incorrect example: My new car came equipped with: power windows, power locks, a sunroof, and a CD player.
b) introduce an appositive at the end of a sentence.
Example: He was shocked at what he saw: his reflection
c) introduce a quotation if it follows a complete sentence.
Example: Mr. Jones used to address our class every day with these words: "Now, settle down, settle down."
d) separate the hour and the minutes in a time reference.
Example: The train is due at 3:35 this afternoon.
e) separate a title from a subtitle.
Example: I am writing Lost and Found: A Chronicle of a Mid-Life Crisis.
f) separate chapter and verse of a scriptural reference.
Example: Today's lesson comes from Psalms 2:1.
g) indicate proportions and ratios.
Example: Mix the oil and vinegar at a 2:1 ratio.
h) substitute for a semicolon between two independent clauses if the second clause further explains the first, or provides an example. (If the material following the colon is an independent clause, it may or may not be capitalized. Choose one method and be consistent.)
Example: The directions to the office are easy: once you have taken the train to Penn Station, go upstairs and grab a cab downtown to 9th street.
More punctuation guides
Writing & Reading Tutoring webpage