A colon means “that is to say” or “here’s what I mean.” Colons and semicolons should never be used interchangeably.
Rule 1. Use a colon to introduce a series of items. Do not capitalize the first item after the colon (unless it’s a proper noun).
Examples:
You may be required to bring many things: sleeping bags, pans, utensils, and warm clothing.
I want the following items: butter, sugar, and flour.
I need an assistant who can do the following: input data, write reports, and complete tax forms.
Rule 2. Avoid using a colon before a list when it directly follows a verb or preposition.
Incorrect: I want: butter, sugar, and flour.
Correct:
I want the following: butter, sugar, and flour.
OR
I want butter, sugar, and flour.
Incorrect: I’ve seen the greats, including: Barrymore, Guinness, and Streep.
Correct: I’ve seen the greats, including Barrymore, Guinness, and Streep.
Rule 3. When listing items one by one, one per line, following a colon, capitalization and ending punctuation are optional when using single words or phrases preceded by letters, numbers, or bullet points. If each point is a complete sentence, capitalize the first word and end the sentence with appropriate ending punctuation. Otherwise, there are no hard and fast rules, except be consistent.
Examples:
I want an assistant who can do the following:
- input data
- write reports
- complete tax forms
The following are requested:
- Wool sweaters for possible cold weather.
- Wet suits for snorkeling.
- Introductions to the local dignitaries.
These are the pool rules:
- Do not run.
- If you see unsafe behavior, report it to the lifeguard.
- Did you remember your towel?
- Have fun!
Rule 4. A colon instead of a semicolon may be used between independent clauses when the second sentence explains, illustrates, paraphrases, or expands on the first sentence.
Example: He got what he worked for: he really earned that promotion.
If a complete sentence follows a colon, as in the previous example, it is up to the writer to decide whether to capitalize the first word. Capitalizing a sentence after a colon is generally a judgment call; if what follows a colon is closely related to what precedes it, there is no need for a capital.
Note: A capital letter generally does not introduce a simple phrase following a colon.
Example: He got what he worked for: a promotion.
Rule 5. A colon may be used to introduce a long quotation. Some style manuals say to indent one-half inch on both the left and right margins; others say to indent only on the left margin. Quotation marks are not used.
Example: The author of Touched, Jane Straus, wrote in the first chapter:
Georgia went back to her bed and stared at the intricate patterns of burned moth wings in the translucent glass of the overhead light. Her father was in “hyper mode” again where nothing could calm him down.
Rule 6. Use a colon rather than a comma to follow the salutation in a business letter, even when addressing someone by his or her first name. (Never use a semicolon after a salutation.) A comma is used after the salutation in more informal correspondence.
Formal: Dear Ms. Rodriguez:
Informal: Dear Dave,
http://www.grammarbook.com/punctuation/colons.asp
