Comma

1. Separates the clauses of a compound sentence connected by a coordinating conjunction: A difference exists between the musical works of Handel and Haydn, and it is a difference worth noting.  The comma may be omitted in short compound sentences: I heard what you said and I am furious. I got out of the car and I walked and walked.
2. Separates and or or from the final item in a series of three or more (optional): Red, yellow, and blue may be mixed to produce all colors.
3. Separates two or more adjectives modifying the same noun if and could be used between them without altering the meaning: a solid, heavy gait. But: a polished mahogany dresser.
4. Sets off nonrestrictive clauses or phrases (i.e., those that if eliminated would not affect the meaning of the sentences): The burglar, who had entered through the patio, went straight to the silver chest.  The comma should not be used when a clause is restrictive (i.e., essential to the meaning of the sentence): The burglar who had entered through the patio went straight to the silver chest; the other burglar searched for the wall safe.
5. Sets off words or phrases in apposition to a noun or noun phrase: Plato, the famous Greek philosopher, was a student of Socrates.  The comma should not be used if such words or phrases precede the noun: The Greek philosopher Plato was a student of Socrates.
6. Sets off transitional words and short expressions that require a pause in reading or speaking: Unfortunately, my friend was not well traveled. Did you, after all, find what you were looking for? I live with my family, of course.
7. Sets off words used to introduce a sentence: No, I haven't been to Paris. Well, what do you think we should do now?
8. Sets off a subordinate clause or a long phrase that precedes a principal clause: By the time we found the restaurant, we were starved. Of all the illustrations in the book, the most striking are those of the tapestries.
9. Sets off short quotations and sayings: The candidate said, “Actions speak louder than words.” “Talking of axes,” said the Duchess, “chop off her head.”—Lewis Carroll
10. Indicates omission of a word or words: To err is human; to forgive, divine.
11. Sets off the year from the month in full dates: Nicholas II of Russia was shot on July 16, 1918.  But note that when only the month and the year are used, no comma appears: Nicholas II of Russia was shot in July 1918.
12. Sets off city and state in geographic names: Atlanta, Georgia, is the transportation center of the South. 34 Beach Drive, Bedford, VA 24523.
13. Separates series of four or more figures into thousands, millions, etc.: 67,000; 200,000.
14. Sets off words used in direct address: “I tell you, folks, all politics is applesauce.”—Will Rogers. Thank you for your expert assistance, Dolores.
15. Separates a tag question from the rest of a sentence: You forgot your keys again, didn't you?
16. Sets off sentence elements that could be misunderstood if the comma were not used: Some time after, the actual date for the project was set.
17. Follows the salutation in a personal letter and the complimentary close in a business or personal letter: Dear Jessica, Sincerely yours, Fred.
18. Sets off titles and degrees from surnames and from the rest of a sentence: Walter T. Prescott, Jr.; Gregory A. Rossi, S.J.; Susan P. Green, M.D., presented the case.

See also: The Comma: A Major Player.