Rules for comma usage:

• Use a comma to separate items in an enumeration.
  Examples:
  Mrs. B had a dog, cat, fish, and a pig. (the comma after fish is optional)
  We use chalk, pencils, pens, and paper in school. (the comma after pens is optional)

• Don’t use a comma before and if two items are a unit; for example, ham and eggs as a dish, is a unit and should not be separated by a comma.
  Examples:
  Mrs. B had soup, ham and eggs, juice, and an apple pie for lunch.
  You have a choice of three breakfasts, namely, ham and eggs, peanut butter and jelly, and pancakes and syrup. (Here the comma after 'jelly' is necessary.)
  Don't run, skip, jump up and down, or crawl in the hall.

• Don't use a comma if all items in an enumeration are separated by and, or, nor etc.
  Example: Mrs. B had a pig and a dog and a cow and a horse.