A colon means "that is to say" or "here's what I mean." Colons and semicolons should never be used interchangeably.
Rule 1a. Use a colon to introduce an item or a series of items. Do not capitalize the first item after the colon (unless it's a proper noun).
You know what to do: practice.
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 1b. A capital letter generally does not introduce a word, phrase, or incomplete sentence following a colon.
He got what he worked for: a promotion
He got what he worked for: a promotion that paid a higher wage.
Rule 2. Avoid using a colon before a list if it directly follows a verb or preposition that would ordinarily need no punctuation in that sentence.
Not recommended: I want: butter, sugar, and flour.
Recommended: I want butter, sugar, and flour.
Here is what I want: butter, sugar, and flour.
Not recommended: I've seen the greats, including: Barrymore, Guinness, and Streep.
Recommended: 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.
I want an assistant who can do the following:
The following are requested:
These are the pool rules:
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, authorities are divided over whether to capitalize the first word. Some writers and editors feel that capitalizing a complete sentence after a colon is always advisable. Others advise against it. Still others regard it as a judgment call: If what follows the colon is closely related to what precedes it, there is no need for a capital. But if what follows is a general or formal statement, many writers and editors capitalize the first word.
Example: Remember the old saying: Be careful what you wish for.
Rule 5. Capitalize the first word of a complete or full-sentence quotation that follows a colon.
Example: The host made an announcement: "You are all staying for dinner."
Rule 6. Capitalize the first word after a colon if the information following the colon requires two or more complete sentences.
Example: Dad gave us these rules to live by: Work hard. Be honest. Always show up on time.
Rule 7. If a quotation contains two or more sentences, many writers and editors introduce it with a colon rather than a comma.
Example: Dad often said to me: "Work hard. Be honest. Always show up on time."
Rule 8. For extended quotations introduced by a colon, 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 9. 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.
Dear Ms. Rodriguez: