Commas & "and"

Q. What is the Oxford comma?
A. The Oxford (or serial) comma is a comma before “and” in a list of things

• I like apples, bananas, and oranges (The Oxford comma is the red comma in the example at left.)
  

Q. Should I use the Oxford comma?
A. Technically the Oxford comma is neither right nor wrong, but some say it clutters the copy. Although, it does have its uses for clarity:

• These shoes come in black and white, red and pink and blue and green. (Huh?)
• These shoes come in black and white, red and pink, and blue and green. (Oh, right.)
• I like my coworkers, George Clooney and Brad Pitt. (This implies my coworkers are movie stars, which is not true.)
• I like my coworkers, George Clooney, and Brad Pitt (This implies I like both my coworkers and a pair of movie stars, which is true.)

Q. When do I put a comma before “and” in a sentence?
A. When connecting two independent clauses

Q. Oh crud. I forgot what independent clauses are. How can I tell if I have two?
A. Independent clauses are parts of a sentence that can stand alone. A good rule of thumb I use is if you introduce a second subject in the second half of the sentence, you use a comma before “and.”

• These boots have a fold-over cuff, and they are also waterproof. (Two subjects)
  
• These boots have a fold-over cuff and are also waterproof. (Only one subject)
  
• I went to the supermarket to buy pecans to bake a pie for next Thursday and ran into my boss. (This is long and confusing but still containing only one subject.) 
  
• I went to the supermarket to buy pecans to bake a pie for next Thursday, and I ran into my boss. (This same subject is mentioned twice, so it takes the comma.)
  

Fun fact! The Oxford comma was first introduced as rule in the 1905 edition of the Oxford University Press Style Guide, which is how it got the nickname. It is sometimes also called the Harvard comma since the Harvard press adheres to it as well.